On February 28, 2022, I presented a pared-down version of this research on the Petoskey Stone, Michigan’s state stone, to the Ozark Mountain Gem & Mineral Society. During my research, I discovered that much of the information available—whether online, in books, or even in magazines—was inaccurate or inconsistent. Determined to separate fact from fiction, I spent four months analyzing sources, cross-referencing data, and compiling the most reliable information into one comprehensive resource.

Petoskey Stones

What is a Petoskey Stone?

Fossilized Colonial Rugose Coral

A Petoskey Stone is a fossilized colony of rugose coral, an extinct group of corals that thrived during the Devonian Period. These corals, which once lived in Michigan’s ancient tropical seas, are now preserved in limestone deposits, their hexagonal patterns making them instantly recognizable.

Understanding Corals and Rugose Corals

Corals are marine invertebrates that have existed from the Ordovician Period (about 485 million years ago) to today, with a 14-million-year gap during the Triassic. They are simple organisms consisting of a polyp, a hollow, cylindrical sac anchored to the seafloor, with a mouth surrounded by tentacles. Unlike more complex animals, coral lack a central nervous system and internal organs. Instead, they extract calcium carbonate (CaCO₃) from seawater to build external skeletons for protection.

Rugose (“Wrinkled”) Corals

Existed from the Ordovician to the Permian.
Lived as solitary individuals or in colonial formations.
Individual corallites (skeleton cups housing polyps) remained small, but entire colonies could grow several feet across.
Likely had stinging tentacles used to capture prey, similar to modern reef-building corals.

Did Rugose Corals Have Symbiotic Algae?

Scientists debate whether rugose corals had a symbiotic relationship with algae. Unlike tabulate corals, which were primary reef-builders, rugose corals played a more minor role in reef formation. In modern corals, this relationship works as follows:

The coral provides a home for algae.
The algae perform photosynthesis using the coral’s waste products.
This process produces oxygen and carbohydrates, benefiting the coral.

Corals are susceptible to temperature—not because they can’t survive in cold water, but because the algae can’t. Most reef-forming corals require temperatures between 73°–84°F (23°–29°C) and clear water for sunlight penetration. Some corals, like those in deeper, colder waters, have adapted to life without algae, but this is a slow evolutionary process.

Geologic Time Scale of the Phanerozoic

Source: Geologic Time Scales

Illustration by Mary Williams of a Colonial Rugose Coral head based on fossils from Thornton Quarry.

Source: Cnidaria: Rugose corals

CNID 3493 Acropora species 3, Matamanoa, Fiji.

Source: Ryan Photographic

Coral Terminology

Understanding the structure of rugose corals, including the Petoskey Stone, requires familiarity with key anatomical terms. Below is a breakdown of essential coral features:

Polyp: The living coral animal, consisting of a soft body, mouth, tentacles, and gullet. Since polyps are made of soft tissue, they do not fossilize.
Corallite: The skeletal cup secreted by the polyp, where it lived and could retract for protection.
Calyx, calice (pl. calyxes, calices): The central part of the corallite where the polyp resided.
Columella: The center of the calyx, where the septa intertwine or form a dome-like or pillar-like structure.
Septum (pl. septa): Radiating support plates within the calyx that provide structural integrity.
Tabula (pl. tabulae): As the polyp aged, it continuously deposited calcium carbonate, raising the walls. When buried under sediment, it built floor-like layers beneath itself to keep growing.
Dissepiment: Curved support plates that connect the septa and tabulae, enhancing vertical stability.
Carina (pl. carinae): A vertical bar found on the septa.

To fully appreciate Petoskey Stones, we need to understand the world where these coral ancestors thrived. The Devonian Period, often called the “Age of Fishes,” was a time of significant marine expansion and biological diversification.

Corallite

Calyx

Columella

Septa

Tabula

Dissepiment

Carinae

Devonian Period

Laurussia: The Devonian Supercontinent

During the Devonian Period (419–359 million years ago), North America, Greenland, and Europe were part of a massive supercontinent called Laurussia, located along the equator and surrounded by warm, shallow seas. The area that is now Michigan’s Lower Peninsula was completely submerged under these tropical waters.

Evolution of Terrestrial Life

The Devonian was a time of dramatic biological advancements, marking the transition of life from water to land:

The first trees emerged, growing up to 10 meters (33 feet) tall.
Early tetrapods (four-legged vertebrates) began to venture onto land.
The first terrestrial arthropods, such as ancient insects and millipedes, made their appearance.

Marine Life & Reef Formation

Devonian oceans were teeming with diverse marine life, including:

A rapid expansion of fish, ranging from jawless armored species to the first lobe-finned fish, the ancestors of modern amphibians.
Corals and sponges formed the first extensive reef systems, which played a crucial role in:
- Protecting coastlines from 97% of wave action, reducing erosion.
- Providing habitats and shelter for a variety of marine species.

However, despite the rich biodiversity of the Devonian seas, this period also saw a series of devastating mass extinctions that would dramatically alter marine ecosystems—including the decline of rugose corals.

Location of Michigan during the Middle Devonian

Source: Petoskey Stone, Finding, Identifying, and Collecting Michigan’s Most Storied Fossil by Dan R. Lynch

Michigan Basin underwater during middle Devonian

Map showing underwater portions of the East and Midwest during the Middle Devonian

Source: The Rapidian

Archaeopteris

Source: Wikipedia

Dunkleosteus

Source: Wikipedia

Devonian Seabed

Click on the link below to access additional links for each number
Source: University of Michigan Museum of Paleontology

Devonian Mass Extinctions

Mass extinctions have shaped life on Earth, with five significant events being the most catastrophic. These occurred at the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods. Each was triggered by dramatic atmospheric chemistry and climate shifts, wiping out vast numbers of species.

The Devonian Extinctions: A Slow Crisis

Unlike some mass extinctions that happened abruptly, the Devonian Period (419–359 million years ago) experienced a series of extinctions over millions of years. These events eliminated 70-80% of all animal species, making it the smallest of the “Big Five” but still devastating—particularly for marine life. One of these extinctions nearly wiped out all coral species between the Middle and Upper Devonian.

What Caused the Devonian Extinctions?

Geologists have identified a layer of black shale in Devonian rock formations, indicating that low-oxygen environments played a key role in these extinctions. The most widely accepted theory links these events to the evolution of deep-rooted land plants, drastically altering Earth’s climate and ecosystems.

Step-by-Step Process of Environmental Collapse

Expansion of Deep-Rooted Plants
- The rise of large, vascular plants accelerated silicate weathering and soil formation.
Runoff and Carbon Sequestration
- As nutrients and carbonates washed from the land into rivers and seas, carbon was trapped in sediments instead of being recycled into the atmosphere.
- This led to a drop in atmospheric CO₂, cooling global temperatures.
Algal Blooms & Oxygen Depletion
- Excess nutrients fueled massive algal blooms.
- These algae blocked sunlight in deeper waters and, upon decaying, consumed oxygen, creating dead zones.
Coral Bleaching & Ecosystem Collapse
- Cooler temperatures and reduced sunlight disrupted coral-algae symbiosis.
- Corals expelled their symbiotic algae (zooxanthellae), a process known as bleaching.
- While some corals can adapt to cooler conditions, oxygen depletion was too rapid for survival.

Many Devonian coral species vanished during these extinctions, but their skeletal remains endured, slowly transforming into limestone over millions of years. This process is key to understanding how Petoskey Stones formed.

Geologic timeline with mass extinctions marked

Mass Extinctions on the Geologic Timeline

Source: University of Kansas

Modern Algal Bloom on Lake Ontario

Source: The Wildlife Society

Mass Extinction Causes Chart

Adapted from: Berkeley

Modern Coral Bleaching

Source: The Guardian

How Coral Turns into Limestone

When a coral colony dies, it becomes buried beneath layers of sediment on the seafloor. The calyx (skeletal cup) fills with sediment as the soft polyp tissue decays. Over time, additional layers of sediment, marine debris, and calcium carbonate accumulate, compacting under pressure.

This process, known as lithification, gradually cements the coral skeletons and other marine remains, transforming them into limestone. This is how Petoskey Stones—fossilized rugose corals—formed over millions of years in what is now Michigan.

But how did these fossilized corals end up in Michigan? To answer that, we need to look at the shifting landscape of Earth’s crust and the movement of continents over time.

Depiction of how coral turns into limestone

How Coral Turns to Limestone

Source: Petoskey Stone, Finding, Identifying, and Collecting Michigan’s Most Storied Fossil by Dan R. Lynch

Michigan on the Move

Tectonic Plate Movement

Earth’s outer layer, the crust, is divided into 15 to 20 tectonic plates, which rest atop the partially molten mantle. These plates are constantly in motion, shifting between 2 to 15 centimeters per year due to radioactive processes within Earth’s interior. Their movement drives earthquakes, volcanic activity, and mountain formation:

When plates slide past each other, they generate earthquakes.
When plates collide, one plate may subduct beneath the other, forming mountain ranges.

Tracking Michigan’s Ancient Location

Geologists reconstruct the movement of tectonic plates using paleomagnetic data, which records the ancient positions of rocks. Scientists can determine when these landmasses were once connected by comparing matching rock formations on separate continents. Additionally, the continental shapes fit together like puzzle pieces, further supporting plate movement over time.

Michigan’s Journey Through Time

Middle Devonian (~385 million years ago) – Michigan was located south of the equator, submerged beneath a warm, tropical sea.
Late Carboniferous (~300 million years ago) – The seas retreated, exposing Michigan as a swampy, semi-tropical jungle. Trees in these ancient wetlands were buried over time, forming coal deposits.
Permian (~280 million years ago) – All landmasses merged into the supercontinent Pangea. Michigan was far inland, preventing further marine fossilization.
Permian to Quaternary (~280 to 2.6 million years ago) – Michigan remained above sea level, limiting fossil preservation due to erosion. This is why few fossils exist from this period, apart from some spore fossils.

While plate tectonics positioned Michigan where it is today, another powerful force—glaciation—played a crucial role in shaping its landscape and exposing the fossils we now find as Petoskey Stones.

Map of the Tectonic Plates

Source: USGS

Pangea Map

Source: Wikipedia

45th parallel marked on northern Michigan map

45th Parallel through Michigan

Adapted from: Google Maps

Quaternary Ice Coverage

During the Quaternary Period (2.6 million years ago), Michigan was entirely covered by ice during all four major North American glaciations. These glaciers reshaped the landscape, carving out valleys, depositing sediments, and forming many of the current geological features.

The Power of Advancing Glaciers

As glaciers advanced southward, they acted like colossal sandpaper sheets, scraping across the landscape and eroding bedrock. This grinding action smoothed out the terrain and carved deep basins, many of which would later fill with water to become Michigan’s iconic lakes. The weight and movement of the ice transformed the region, reshaping river courses and sculpting hills.

Retreating Ice and the Legacy of Deposition

When the climate warmed, the glaciers began to retreat, leaving vast amounts of glacial till—a mixture of sand, gravel, clay, and boulders. This debris settled into the landscape differently, creating rolling hills and expansive plains in some areas while leaving other regions with exposed bedrock.

One of the most significant results of glacial retreat was the formation of moraines—long, ridge-like accumulations of debris that mark where glaciers paused during their retreat. These features still shape Michigan’s topography today.

The Birth of the Great Lakes

As the glaciers retreated for the final time, they left behind huge depressions carved into the bedrock. Over time, these basins filled with meltwater, forming what we now know as the Great Lakes—some of the largest freshwater bodies on Earth. Many of Michigan’s smaller lakes and wetlands also owe their existence to these ancient glaciers.

The thickness of glacial till across Michigan varies dramatically:

Coastal areas have thinner deposits, where bedrock is closer to the surface.
Inland regions have thicker layers of till, forming features such as moraines and drumlins—elongated hills that trace the movement of the ice.

Today, Michigan’s landscape remains a testament to the immense power of glaciers, with its rolling terrain, fertile plains, and freshwater lakes all shaped by the ice that once covered the land.

Before we dive deeper into how to identify a Petoskey Stone, let’s explore its scientific classification and how experts have defined these fossils over the years.

Extent of Pleistocene Ice Sheets in the Northern United States

Source: Encyclopaedia Britannica

Depiction of glaciers moving ice from the north to the southwest

Direction of Glacial Flow

Source: Illinois State Geological Survey

Michigan Bedrock Map

Source: USGS
NOTE: Most Michigan Bedrock maps show Jurassic Red Beds in the middle of the state, they are more likely Pennsylvanian, and I have modified the key to show this.

Michigan Glacial Till

Source: Department of Environmental Quality

Scientific Classification

Like many fossils, the Petoskey Stone has undergone multiple taxonomic reclassifications as scientists have improved their understanding of its structure. Over time, researchers have refined its classification based on diagnostic features observed in Michigan specimens.

Historical Taxonomic Changes

1866 – Newton Horace Winchell
- Acervularia davidsoni (based on the 1855 work of Henri Milne-Edwards and Jules Haime).
1876 – Carl Ludwig Rominger
- Reclassified as Cyathophyllum davidsoni after determining that Acervularia required a central portion of polyp cells surrounded by an internal wall, which Michigan specimens lacked.
1935 – William Dickson Lang & Stanley Smith
- Reassigned from Cyathophyllum davidsoni to Prismatophyllum davidsoni, introducing new diagnostic criteria to differentiate Michigan specimens.
1939 – Laurence L. Sloss
- Renamed Prismatophyllum percarinatum, distinguishing it from P. davidsoni (France), which lacked carinae (ridge structures).
1970 – Erwin C. Stumm
- Hexagonaria percarinata – The most widely accepted modern classification.

Within Michigan, several species of Hexagonaria have been identified. Understanding these variations can help collectors and enthusiasts distinguish between different types of Petoskey Stones.

Hexagonaria Taxonomy

Source: Interim Register of Marine and Nonmarine Genera

Alexander Winchell

Source: University of Michigan

Carl Ludwig Rominger

Source: University of Michigan

Laurence L. Sloss

Source: Geological Society of America

Erwin C. Stumm

Source: University of Michigan

Michigan Hexagonaria Species

Erwin C. Stumm’s Contributions

Dr. Erwin C. Stumm was a Professor of Geology and Curator of Paleozoic Invertebrates at the University of Michigan, widely regarded as a leading expert on Devonian invertebrate fossils. His research was prolific, with 56 published papers and his name cited in over 5,947 scholarly references.

One of his most significant works, Corals of the Traverse Group of Michigan, was a 13-part series (1949–1969) documenting the Devonian coral fauna of northern Michigan. Unfortunately, Stumm passed away before completing the entire series—his final contribution, Part 13: Hexagonaria, was published just 15 days before his death.

Hexagonaria Species in Michigan

In his research, Stumm identified nine distinct species of Hexagonaria from fossil specimens previously collected by Laurence L. Sloss and G.M. Ehlers. Each species is associated with specific Devonian rock formations in Michigan:

Hexagonaria alpenensis: Alpena Limestone
Hexagonaria anna: Bell Shale, upper Ferron Point Formation
Hexagonaria attenuata: Alpena Limestone, Four Mile Dam Formation, Charlevoix Limestone
Hexagonaria cristata: Gravel Point Formation
Hexagonaria fusiformis: Genshaw Formation
Hexagonaria percarinata: Gravel Point Formation
Hexagonaria potterensis: Potter Farm Formation, Thunder Bay Limestone
Hexagonaria profunda: one specimen from Petoskey Limestone
Hexagonaria subcarinata: Alpena Limestone

Additionally, Stumm provided a chart of eight diagnostic characteristics used to differentiate these species.

Characteristics of Michigan Hexagonaria Species

Source: Corals of the Traverse Group of Michigan, Part 13 Hexagonaria

Stratigraphy

What is Stratigraphy?

Stratigraphy is the branch of geology that studies rock layers’ order, relative position, and age (strata). Geologists can classify mappable rock units by examining rock sequences and determining their geologic timeframes.

The Michigan Basin

Michigan’s Lower Peninsula is centered on a large geologic basin known as the Michigan Basin. This bowl-shaped depression is deepest in the center and rises toward the coasts, explaining why older rock layers are more exposed along Michigan’s shorelines than in its interior.

Hexagonaria & Devonian Strata

Fossilized Hexagonaria corals (including the Petoskey Stone) are only found in situ (in their original rock layers) within the Traverse Group and Bell Shale formations in northern Michigan.

While other Middle Devonian rock layers exist throughout the state, including around Detroit, Hexagonaria fossils are only present in formations north of central Michigan.

Traverse Group Formations

The Traverse Group, which contains many of Michigan’s Devonian coral fossils, consists of over 20 formations and is divided into three geographic regions:

Northwestern Region – Emmet & Charlevoix Counties
North Central Region – Afton to Onaway
Northeastern Region – Presque Isle & Alpena Counties

Michigan Basin

Source: State of Michigan

Northern Michigan County Map

Source: Randy Majors

Traverse Group Locations

Source: Devonian Strata of Northern Michigan

Traverse Group Formation Locations Northwest Lower Peninsula Michigan

Northwest Michigan

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Traverse Group Formation Locations North Central Lower Peninsula Michigan

North Central Michigan

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Traverse Group Formation Locations Northeast Lower Peninsula Michigan

Northeast Michigan

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Hexagonaria alpenensis

Formation & Location

Formation: Alpena Limestone
Region: Northeast Lower Peninsula, Michigan

Description

Found in Alpena Limestone, which consists of white, light gray, or light brown rock with many bioherms (reef-like structures).
Displays extreme compositional variability compared to other Hexagonaria species.
The smallest species of Hexagonaria identified in Michigan.

Alpena Limestone

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Hexagonaria alpenensis

Source: Corals of the Traverse Group of Michigan Part 13, Hexagonaria

Hexagonaria anna

Formation & Location

Formations:
- Bell Shale
- Rockport Quarry Limestone
- Ferron Point Formation
Region: Northeast Lower Peninsula, Michigan
Additional Occurrences: Found near Antwerp, Ohio

Formation Descriptions

Bell Shale (BS) – Gray-blue, fossiliferous clay shale, soft and readily disintegrating.
Rockport Quarry Limestone (RQL) – Gray to brown limestone, sublithographic, with yellow calcite specks.
Ferron Point Formation (FPF) – Greenish-gray clay, forming a densely packed surface.

Bell Shale, Rockport Quarry Limestone, and Ferron Point Formation

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Hexagonaria anna

Source: Corals of the Traverse Group of Michigan Part 13, Hexagonaria

Hexagonaria attenuata

Formation & Location

Formations & Regions:
- Alpena Limestone – Northeast Lower Peninsula, Michigan
- Four Mile Dam Formation – Northeast Lower Peninsula, Michigan
- Charlevoix Limestone – Northwest Lower Peninsula, Michigan
Key Exposure Sites:
- Kegomic Quarry Syncline – Located at the eastern limit of Emmet County in the Bear Creek Valley, where the Petoskey Formation is exposed above the Charlevoix Limestone at Bay View.
- Only Hexagonaria species found on both coasts of Michigan’s Lower Peninsula.

Formation Descriptions

Alpena Limestone (AL) – White, light gray, or light brown, highly variable composition with numerous bioherms (reef-like structures).
Four Mile Dam Formation (FMD) – Brownish-gray limestone, interbedded with sandy shale partings.
Charlevoix Limestone (CL) – Found in northwestern Michigan, prominently exposed in the Kegomic Quarry Syncline.

Distinctive Features

Similar to Hexagonaria subcarinata, but with:

Longer major septa
Smaller corallites
Lack of a distinct boundary between the dissepimentarium and tabularium in transverse section

Charlevoix Limestone

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Alpena Limestone, Four Mile Dam Formation

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Hexagonaria attenuata

Source: Corals of the Traverse Group of Michigan Part 13, Hexagonaria

Hexagonaria cristata

Formation & Location

Formation: Gravel Point Formation
Region: Northwest Lower Peninsula, Michigan
Primary Exposure Site: Little Traverse Bay

Formation Characteristics

Found within a 10–15 ft limestone interval between the lower and upper blue shales.
Associated with the Gravel Point Formation, which is known for well-preserved Devonian marine fossils.

Distinctive Features

Largest species of Hexagonaria identified in Michigan.

Gravel Point Formation

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Hexagonaria cristata

Source: Corals of the Traverse Group of Michigan Part 13, Hexagonaria

Hexagonaria fusiformis

Formation & Location

Formations & Region:

Genshaw Formation – Northeast Lower Peninsula, Michigan
Ferron Point Formation – Northeast Lower Peninsula, Michigan
Newton Creek Limestone – Northeast Lower Peninsula, Michigan

Formation Descriptions

Genshaw Formation (GF) – Gray to light brown limestone, occasionally containing small bioherms (reef-like structures).
Ferron Point Formation (FPF) – Greenish-gray clay, forming a densely packed surface.
Newton Creek Limestone (NCL) – Dark brown crystalline limestone.

Distinctive Features

Similar to Hexagonaria anna, but differs by:

Longer rhopaloid major septa
More distinctly zigzag corallite walls

Genshaw Formation, Ferron Point Formation, and Newton Creek Limestone

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Hexagonaria fusiformis

Source: Corals of the Traverse Group of Michigan Part 13, Hexagonaria

Hexagonaria potterensis

Formation & Location

Formations & Region:
- Potter Farm Formation – Northeast Lower Peninsula, Michigan
- Thunder Bay Limestone – Northeast Lower Peninsula, Michigan
Key Exposure Site:
- Thunder Bay Limestone is exposed at or just below water level at Partridge Point.

Formation Descriptions

Potter Farm Formation (PFF) – Brownish-gray limestone with sandy shale partings.
Thunder Bay Limestone (TBL) – Bluish, argillaceous limestone that weathers to rusty brown.

Distinctive Features

Calyx walls are intermediate between Hexagonaria percarinata and Hexagonaria profunda.

Potter Farm Formation & Thunder Bay Limestone

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Hexagonaria potterensis

Source: Corals of the Traverse Group of Michigan Part 13, Hexagonaria

Hexagonaria profunda

Formation & Location

Formation: Petoskey Limestone
Region: Northwest Lower Peninsula, Michigan
Key Exposure Site: Kegomic Quarry

Formation Description

Petoskey Limestone (PL) – Gray, shalelike limestone.
Only one specimen of H. profunda has been found in Michigan.
Identical to specimens from the Cedar Valley Limestone in Iowa.

Distinctive Features

Extremely rare in Michigan, making it unlikely to be encountered in the state.

Petoskey Limestone

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Hexagonaria profunda

Source: Corals of the Traverse Group of Michigan Part 13, Hexagonaria

Hexagonaria subcarinata

Formation & Location

Formation: Alpena Limestone
Region: Northeast Lower Peninsula, Michigan

Formation Description

Alpena Limestone consists of white, light gray, or light brown rock, often containing numerous bioherms (reef-like structures).
Highly variable composition compared to other Devonian formations.

Distinctive Features

Very similar to Hexagonaria percarinata, but:

Lacks abundant carinae (ridge-like structures).
Less likely to be found embedded in shale.

Alpena Limestone

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Hexagonaria subcarinata

Source: Corals of the Traverse Group of Michigan Part 13, Hexagonaria

Hexagonaria percarinata

Formation & Location

Formation: Gravel Point Formation
Region: Northwest Lower Peninsula, Michigan
Key Exposure Site: Little Traverse Bay

Formation Description

Found in Gravel Point Formation, composed of brownish-gray limestone with sandy shale interbeds.

Distinctive Features

Most common species of Hexagonaria in Michigan.

Can be distinguished from other species by:

A false inner wall created by the crowding of dissepiments and carinae at the axial ends of the minor septa.
Being much more heavily carinate compared to other Hexagonaria species.

Gravel Point Formation

Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties

Hexagonaria percarinata

Source: Corals of the Traverse Group of Michigan Part 13, Hexagonaria

Hexagonaria percarinata

Source: Corals of the Traverse Group of Michigan Part 13, Hexagonaria

Gravel Point Formation

Location & Exposure

Region: Northwest Lower Peninsula, Michigan
Key Exposure Site: Bay Harbor, Petoskey, Michigan

Formation Description

Gravel Point Formation is a Devonian limestone formation known for its fossil-rich deposits, particularly Hexagonaria percarinata.
The formation is prominently exposed behind Bay Harbor in Petoskey, Michigan, where rock strata can be observed.

Gravel Point Formation

Gravel Point Formation

Worldwide Hexagonaria

Global Distribution

The fossilized coral Hexagonaria is not unique to Michigan—its presence spans the globe. To date, over 70 species of Hexagonaria have been identified in 31 different locations worldwide. From North America to Europe, Asia, and beyond, these corals have been discovered in diverse geological formations, each offering a glimpse into ancient marine ecosystems.

However, as scientific techniques and taxonomic classifications evolve, some species originally identified as Hexagonaria may have undergone reclassification. Advances in paleontology, stratigraphy, and fossil analysis continue to refine our understanding of these corals, sometimes placing them into different genera or identifying previously unrecognized variations.

Despite these changes, the widespread presence of Hexagonaria highlights its success as a coral genus during the Devonian Period, when warm, shallow seas covered much of the Earth. Today, whether found in Michigan’s Petoskey Stones, the limestone of Europe, or the fossil beds of Asia, these ancient corals serve as a reminder of a time when the world was vastly different—and yet, in some ways, still connected by the enduring traces of life.

**Known Hexagonaria Species**

(Note: This list may include synonyms or reclassified taxa)

adarensis
allani
amanshauseri
anna
approximans
arctica
basaltiformis
bassleri
bella
bompasi
bongbutensis
bouchardi
capitolium
caurus
cincta
curta
davidsoni
densa
firthi
fisherae
flexum
gamboni
hexagona
hypocrateriforme
inequalis
isylica
jurkowicensis
kirki
kuznetskiensis
lavali
laxa
longiseptata
mae
magna
marmini
meeki

meoualis
minuta
mirabilis
mireillae
occidens
orientalis
ovoidea
oweni
pachytheca
palmera
parallaxa
partita
parvula
pentagona
percarinata
philomena
playfordi
ponderosa
prisma
quadrigemina
reedi
rohrensis
sanctacrucensis
schucherti
septaforminalis
soraufi
stenotabulata
stewartae
subcarinata
tabulata
taurensis
truncata
tungkanlingensis
tuqiaoziensis
whitfieldi
yakovlevi

Known Geographic Locations

Hexagonaria species have been found in:

North America:
- Alaska, Alberta, Arizona, Indiana, Iowa, Kentucky, Manitoba, Michigan, New Mexico, New York, Northwest Territories, Nunavut, Ohio, Ontario, Yukon
Europe:
- Belgium, France, Germany, Poland, Spain, United Kingdom
Asia & Middle East:
- Afghanistan, China, Iran, Kazakhstan, Tajikistan, Vietnam

Oceania:
- Australia, New Zealand
Africa:
- Morocco
Russia & Former Soviet Regions:
- Russian Federation

World Map with Hexagonaria locations marked

Worldwide Hexagonaria Locations

Adapted from: Google Maps

Where to Look in Michigan

Petoskey Stones can be found scattered throughout Michigan. Still, the best places to hunt for them are along the rocky beaches of the Lower Peninsula, in glacial deposits, and within limestone quarries. While they may occasionally turn up in unexpected locations, certain areas are particularly well known for yielding these fossilized corals.

The Best Places to Look

If you’re searching for Petoskey Stones, the easiest places to find them are rocky beaches along Michigan’s Lower Peninsula. These stones have been naturally tumbled by waves, smoothing their surfaces and making their distinctive hexagonal patterns easier to spot.

Some of the top beach-hunting locations include:

Little Traverse Bay – The best place to find Petoskey Stones.
Grand Traverse Bay – Another excellent spot with high-quality specimens.
Leelanau Peninsula – Well-known for its abundance of Petoskey Stones.
Rogers City to Alpena – A great stretch of coastline where fossilized corals are often found.

If beachcombing isn’t your style, you can also find Petoskey Stones scattered in glacial till and gravel deposits throughout the state. Because glaciers carried these fossils far from their original formations, it’s possible to stumble across them in gravel driveways, construction sites, and garden beds.

Another lesser-known but productive hunting ground is Michigan’s limestone quarries. Northern Michigan was once one of the world’s largest limestone producers, and Devonian fossils, including Petoskey Stones, are frequently uncovered in these quarries.

The Best Time to Look

Finding a Petoskey Stone is partly about knowing when to search. The best times to hunt are:

In the spring, after the ice breakup, as winter ice melts, rocks shift, and new stones are revealed along the shore.
After storms – Waves erode lakebed formations and deposit fresh rocks onto the beaches.

These natural processes constantly replenish the supply of Petoskey Stones, ensuring that patient hunters will always have new fossils to discover.

Why Are Petoskey Stones Found in the Great Lakes?

It might seem odd to find coral fossils along Michigan’s lakeshores, given that corals thrive in warm, tropical waters. However, the Great Lakes did not exist when these corals were alive. Instead, during the Devonian Period, Michigan was covered by a shallow tropical sea. Over time, the fossil-bearing formations that once lay beneath this sea extended into the areas that would eventually become the Great Lakes.

As lake waves and ice erode these ancient limestone formations, Petoskey Stones break free and wash up on shore, making them relatively easy to collect.

Rules and Regulations for Collecting Petoskey Stones

Before heading out on a Petoskey Stone hunt, it’s essential to be aware of Michigan’s rock-collecting laws:

On state lands, including state parks, lakes, and public lands, there is a 25-pound per year weight limit for collecting rocks.
National Parks (Sleeping Bear Dunes, Pictured Rocks, and Isle Royale) prohibit rock collecting entirely.

Ignoring these regulations can result in fines, so it’s best to collect responsibly and leave enough fossils behind for future generations to enjoy.

Limestone Quarries & Glacial Deposits

Michigan’s limestone industry has played a significant role in unearthing Devonian fossils. At its peak, Michigan had 31 active limestone quarries (as of 1978), most concentrated in northern Michigan and the eastern Upper Peninsula.

One of the most famous sites is Calcite Quarry in Rogers City, which remains the largest limestone quarry in the world. Limestone from these quarries has been used for road construction, building materials, and cement production. As a result, many fossils—including Petoskey Stones—have been found in gravel and crushed stone used in construction projects.

Additionally, glacial activity helped distribute Petoskey Stones throughout the state, scattering them far beyond their original formations. This means you don’t necessarily need to visit a beach or quarry to find one—they can turn up almost anywhere.

A Fossil Hunter’s Treasure

Whether you find a Petoskey Stone along the shore, in a limestone quarry, or your driveway, each carries a 400-million-year-old story of Michigan’s prehistoric past. Hunting for these unique fossils is not just a rewarding hobby—it’s a way to connect with the land’s ancient history beneath our feet.

Map marking public rock hounding spots in Michigan

Michigan Public Beaches

Source: Michigan Rockhounds

State confiscates 93-pound Petoskey Stone from Michigan Man

Source: Michigan Live

Great Lakes Bedrock Map

Source: National Earth Science Teachers Association

Picture of Calcite Quarry taken from the International Space Station

Calcite Quarry, Roger's City, Michigan

Source: NASA Earth Observatory

Origins of the Name

The Petoskey Stone is often said to be named after the great Ottawa chief Pet-O-Sega, whose name translates to “Rising Sun,” “Rays of Dawn,” or “Sunbeams of Promise.” While this legend is widely told, the more likely explanation is that the stone was named after the city of Petoskey, where it was commonly found and sold as a souvenir. It is possible that someone intentionally connected the name to the chief as a marketing strategy. Still, it is unlikely that the name emerged without knowing the city’s origins.

The Man Behind the Name

Petoskey, Michigan, was named after Biidassige, an Odawa fur trader and businessman born in 1787, whose anglicized name was Ignatius Petoskey (Neyas Petosega). His story is deeply intertwined with Michigan’s history of land treaties, settlement, and displacement of Native American communities.

Bear River/Creek (Mukwa Ziibing) had been a small Odawa fishing village for centuries. In 1836, Petoskey and his sons purchased 440 acres of land where the Bear River meets Lake Michigan. At the time, Native Americans across Michigan were buying back land they had just sold to the U.S. Government through the Treaty of Washington (1836). This treaty had initially promised them permanent hunting and fishing rights, education, monetary compensation, and other services. However, Congress later altered the treaty’s terms, reducing the guaranteed benefits to five years. Many Native Americans, including Petoskey, believed that purchasing land would give them the same rights as white settlers and prevent forced relocation.

Loss of Land and Broken Promises

In 1852, a Presbyterian Mission was established in Bear River when a minister arrived with a deed to 80 acres of the 440 acres Petoskey had purchased. According to Warren Petoskey, Ignatius Petoskey’s great-great-grandson, in his book Dancing the Dream, Petoskey was unaware he needed to pay taxes on the land. The state sold the land to the Presbyterian Church due to unpaid taxes. Still, Petoskey did not challenge the loss, fearing that drawing attention to himself and the Odawa community could result in forced removal to Kansas.

The Treaty of Detroit (1855) sought a solution by granting 40- to 80-acre plots of land to the Odawa within specific townships. These lands were reserved for 10 years and could not be sold or transferred. After this period, any remaining land could be sold by the state. However, exemptions were made for churches, missions, schools, and settlers already living there. When land sales opened to the public, Petoskey and his sons had accumulated most of downtown Bear River.

The Town of Petoskey is Born

In 1873, the Grand Rapids & Indiana Railroad added a rail stop in Bear River, bringing new settlers to the region. Seeing an opportunity for growth, Dr. William Little applied for a post office and requested the town be renamed Petoskey in honor of Ignatius Petoskey.

Unfortunately, Petoskey and his sons lost most of their land over time. Ignatius Petoskey passed away in 1885, but his descendants still live in the area today. In 1965, his granddaughter, Ella Jane Petoskey, co-signed Act 89 of 1965, officially declaring the Petoskey Stone as Michigan’s state stone.

Legacy and Recognition

Today, Petoskey’s name lives on both the city and the fossilized coral, which has become one of Michigan’s most recognizable natural treasures. Whether the stone was named in direct honor of Pet-O-Sega or simply after the city, the Petoskey Stone remains a symbol of Michigan’s geological and cultural history.

Statue of Ignatius Petoskey in Petoskey Harbor

Chief Petosega

Source: Petoskey Chamber of Commerce

Ignatius Petoskey

Source: Little Traverse Historical Society

Map of property owned by Petoskey and his sons in 1879

Land Belonging to Petoskeys when Village of Petoskey was Incorporated in 1879

Adapted from: Google Maps

Ella Jane Petoskey

Source: Warren Petoskey

So, What is a Petoskey Stone?

A Petoskey Stone is the commonly used name for fossilized Hexagonaria corals found in Michigan. While many associate it specifically with northern Michigan, glaciers carried these fossils far beyond their original formations, scattering them throughout the state and even into neighboring regions. Additionally, limestone quarries have supplied an abundance of Petoskey Stones, distributing them in gravel used for roads and construction.

What Defines a “True” Petoskey Stone?

The debate over what qualifies as a “true” Petoskey Stone has persisted for decades. Some argue that only specimens found in northern Michigan should bear the name, dismissing those discovered elsewhere as unrelated. Others insist that only Hexagonaria percarinata, one of several Hexagonaria species in Michigan, deserves the title.

The scientific connection between H. percarinata and the Petoskey Stone can be traced back to Laurence Sloss’s 1937 doctoral dissertation at the University of Chicago. In his research, he referred to the fossil as Prismatophyllum percarinatum, describing it as “the coral of the familiar ‘Petoskey stone’ of northern Michigan.” Given that Sloss was from California and studying in Illinois, his statement suggests that Petoskey Stones were already well-known and widely recognized at the time.

How Common Are Petoskey Stones?

Whether or not Petoskey Stones were sold outside of Michigan in the 1930s remains unclear, but today, they are widely available. Souvenir shops across Michigan sell them, and collectors frequently find them along the shores of Lake Michigan and Lake Huron. Many people don’t realize that there are multiple species of Hexagonaria, and even those who do often lack the scientific expertise or equipment to identify them correctly.

Adding to the complexity, most Petoskey Stones found on Michigan’s beaches originate from bedrock beneath the lakes, rather than the exposed formations geologists use for reference. To date, no large-scale geological study has been conducted to determine which species of Hexagonaria exist in the submerged bedrock, nor have widespread fossil analyses been performed on lake-retrieved specimens.

The Great Petoskey Stone Debate

Disagreements over what constitutes a “true” Petoskey Stone frequently surface, particularly online. Some claim that only fossils from the Gravel Point Formation qualify, yet they contradict themselves by citing examples found in places where that formation is not even exposed, such as Leelanau County.

Others mistakenly believe that Petoskey Stones aren’t found in Petoskey at all, assuming the name derives solely from the “Rising Sun” meaning associated with Chief Petosega. Still, another group insists that their Petoskey Stone must be H. percarinata, despite being unable to distinguish it from similar species like H. subcarinata.

A Practical Approach to Petoskey Stones

Rather than engaging in endless debates over classification, embracing the broader definition is best: any Hexagonaria fossil found in Michigan or the Great Lakes region can reasonably be called a Petoskey Stone. The specific species identification should be left to paleontologists, not casual collectors. Unless you’ve examined a specimen under a microscope or extracted it directly from the Gravel Point Formation along Little Traverse Bay, there’s no way to classify it as H. percarinata definitively.

So, instead of arguing over semantics, let’s appreciate these remarkable fossils for what they are—a tangible connection to a prehistoric marine world beautifully preserved in stone.

For those hoping to find and recognize a Petoskey Stone in the wild, knowing the different species is helpful—but being able to identify one on sight is even more practical.

Visual Identification

Like all limestone, Petoskey Stones are naturally white, but their coloration has been altered over time by environmental factors. The gray and brown tones characteristic of many specimens come from crude oil that once permeated the surrounding rock. Over time, mud and silt filled the coral’s skeletal structures, darkening the “eyes” of the fossilized corallites and creating distinctive hexagonal patterns.

The location where a Petoskey Stone is found dramatically influences its appearance. Stones shaped by the constant tumbling of waves along the Great Lakes coastline tend to be smooth and rounded, their surfaces contoured like polished soap. These beach-worn specimens are typically pebble to fist-sized, with egg-sized being the most common. When dry, they resemble ordinary limestone, but their intricate hexagonal patterns emerge in striking contrast as soon as they become wet.

In contrast, Petoskey Stones found inland, buried in glacial till or exposed in quarries, often retain more of their original structure. These stones are less worn, making the coral’s intricate skeletal details easier to see. Some of these specimens even preserve the stem of the coral, giving them a distinctive mushroom-cap appearance.

Occasionally, a Petoskey Stone will reveal a different side of its structure. Some specimens display tabulae—the internal horizontal layers of the coral—on their surface, as if the coral had grown sideways. Others may be embedded with remnants of additional fossils, evidence of the diverse marine environment they originally formed.

Dry, Unpolished Petoskey Stone

Wet, Unpolished Petoskey Stone

Smooth, Unpolished Petoskey Stones

Rough, Unpolished Petoskey Stones

"Mushroom Caps"

Petoskey Stones with Sides on Top

Petoskey Stones Mixed with Other Fossils

Color Variations

Petoskey Stones are most commonly gray or brown, but they can also display a variety of colors depending on the minerals present during fossilization. While the exact composition of these pigments can only be determined using x-ray fluorescence analysis, the prevailing theory suggests that iron is the primary cause of most color variations.

Ferric oxide (Fe₂O₃), found in minerals like hematite and limonite, produces red, orange, or yellow hues.
Ferrosoferric oxide (Fe₃O₄), commonly known as magnetite, can create a blue coloration.
A mixture of ferrosoferric oxide with a large amount of ferric oxide results in purple tones.
A mixture of ferrosoferric oxide with a small amount of ferric oxide can lead to a greenish coloration.
Ferrous oxide (FeO) is colorless and acts as a diluting agent when mixed with other iron oxides, softening their intensity.

Other possible contributors to Petoskey Stone coloration include copper (which can create blue-green hues), nickel (blue), low-oxygen sediment environments (which may produce blue-gray tones), and manganese (which can contribute to purple hues).

Silicification and Petoskey Stones

In some locations, fossilized corals undergo silicification, a process in which silica minerals replace the original calcium carbonate skeleton. Silicification often enhances the structural details of a fossil, making it more resistant to erosion and easier to extract from surrounding rock, as silica is much harder and does not dissolve in acids.

However, silicified Petoskey Stones are rare. When silicification does occur in these fossils, it happens after their initial fossilization. Unlike in other corals, replacing calcium carbonate with silica tends to obscure rather than enhance the coral’s distinctive hexagonal pattern.

This variety in coloration and preservation makes Petoskey Stones even more intriguing, as each specimen tells a unique story of the chemical and environmental conditions that shaped it over millions of years.

Because of their popularity, Petoskey Stones are often misidentified—or even deliberately misrepresented. Here’s how to spot the real deal.

Brown Petoskey Stones

Gray Petoskey Stones

Multicolored Petoskey Stones

Silicified Petoskey Stones

How to Spot a Fake

Because Petoskey Stone is a widely recognized and marketable name, many fossilized corals worldwide are misidentified or deliberately mislabeled as Petoskey Stones when not Hexagonaria from northern Michigan. While this can be misleading—either through dishonesty or simple lack of knowledge—it doesn’t mean the coral fossil isn’t real, valuable, or beautiful. It simply means it is not the same species or not from Michigan.

To help distinguish authentic Petoskey Stones from other coral fossils, refer to this guide on identifying Devonian corals found in Michigan and different types of corals.

Common Coral Fossils Misidentified as Petoskey Stones

Moroccan Coral Fossils

Coral fossils from Morocco are frequently mislabeled as Petoskey Stones. While Hexagonaria has been recorded in Aferdou el Mrakib, Morocco, I have yet to find an image of a confirmed specimen.

One of the most frequently misidentified Moroccan fossils is Lonsdaleia, a coral that can be easily distinguished from Hexagonaria. The key difference is in the septa—in Lonsdaleia, the septa do not extend to the corallite walls. Instead, they stop at the edge of the calyx, creating a large zone of bubbly dissepiments between the calyx and the corallite walls.

Aferdou el Mrakib, Morocco

Source: Google Maps

Moroccan Lonsdaleia

Acrocyathus – Michigan’s Similar Fossil

Another fossil coral, Acrocyathus, is often confused with Lonsdaleia, but it has a lens-like columella. Acrocyathus fossils can be found in the Mississippian Bayport Limestone in Michigan. While these fossils are found in the state, they are not Hexagonaria and not Petoskey Stones.

Acrocyathus

Indonesian Agatized Coral (Chrysanthemum Coral)

A different type of coral commonly mislabeled as Petoskey Stone is Indonesian agatized coral, a silicified Scleractinian coral. These fossils are typically pink or yellow; some are heat-treated to enhance their colors. This material is often sold as “Chrysanthemum Coral” rather than Petoskey Stone, but mislabeling does occur.

From the specimens I’ve come across, the sellers did not market them as Petoskey Stones, but I’ve seen mislabeled listings.

Artificial “Petoskey Stones”

Not all fake Petoskey Stones are fossilized corals—some are entirely man-made! There are polymer clay replicas designed to mimic the appearance of polished Petoskey Stones. While these are not real fossils, they are impressive works of art demonstrating just how recognizable the Petoskey Stone pattern is.

Final Thoughts

Identifying genuine Petoskey Stones requires an understanding of Hexagonaria’s structure, as well as knowledge of other fossil corals. While other coral fossils can be stunning and scientifically significant, only fossilized Hexagonaria from Michigan should be called Petoskey Stone. When buying or collecting, be aware of these differences and educate others to help preserve the name’s integrity.

Once you’ve found an authentic Petoskey Stone, proper care and polishing can enhance its beauty. Here’s how to do it safely and effectively.

Picture of two agatized Indonesian coral cabochons

Indonesian Agatized Coral

Petoskey Stones in Polymer Clay

How to Clean and Polish

Petoskey Stones are relatively easy to clean and polish, but care must be taken to preserve the delicate fossil structure. Their porous nature makes them prone to absorbing water and other substances, affecting their appearance and polish. Below are the best methods to clean, smooth, and polish Petoskey Stones without damaging them.

Cleaning Petoskey Stones

A soft-bristled toothbrush and vinegar can clean Petoskey Stones, but caution is needed. Vinegar is acidic and will begin dissolving the calcium carbonate of the fossil if left in contact for too long.

Steps to Clean with Vinegar:

Lightly scrub the stone with a soft-bristled toothbrush.
Dip the stone in vinegar for a few seconds—do not leave it submerged for long.
Rinse immediately with plenty of water.
To neutralize any remaining vinegar, create a baking soda bath (a small amount of baking soda dissolved in water) and rinse the stone.
Dry the stone entirely before moving on to polishing.

If you leave a Petoskey Stone in vinegar too long, you will notice a whitish, pitted surface where the acid has eaten away at the fossil. While this might not ruin the stone, polishing will be more difficult.

Sanding and Polishing by Hand

Petoskey Stones sand very quickly, but their porous nature means some spots may be difficult to polish thoroughly. Some stones will have flaws, pits, or divots—sometimes you can sand past them, but sometimes it’s best to accept them as part of the stone’s character.

Recommended Sanding Process:

Grind down cracks or significant imperfections with a grinder (optional).
Start with 60-grit sandpaper to remove any rough spots.
Progress through finer grits: 150 → 320 → 600 → 1000 → 3000 → 5000 → 10,000.
- Spend extra time on the coarser grits, ensuring a smoother finish in the later stages.
- By 3000 grit, you should already see a noticeable shine.
- If polishing isn’t working, back down a few grits and redo those steps.
Final Polish (Optional): Use cerium oxide or aluminum oxide polishing compound with a soft cloth.

With this method, there is no need to soak the stones in mineral oil or coat them in epoxy. Proper sanding and polishing will naturally bring out the beautiful hexagonal patterns.

Using a Rock Tumbler

Petoskey Stones can be tumbled, but results vary. They are soft and porous, which means they can erode too much if tumbled aggressively. I prefer to tumble them for the initial shaping stages and switch to hand sanding.

Recommended Tumbler Process:

Start with 320 or 500-grit silicon carbide and add a thickening agent to create a slurry.
- Some people use syrup, sugar, or molasses, but I prefer guar gum to avoid a sticky mess.
Use 600 or 800-grit silicon carbide with a 1:1 or 2:1 media ratio to stones.
- Recommended media: walnut shells, corn cob, or cut-up neoprene foam (I use foam).
For finer smoothing, use 600-grit aluminum oxide.
Polish with cerium oxide or 0.5–0.8 micron aluminum oxide mixed with media and thickener.

What NOT to Do

Do not coat Petoskey Stones with epoxy, polyurethane, or nail polish—this creates a fake-looking glossy finish that obscures the natural beauty of the fossil.
Avoid soaking them in vinegar or acids for long periods—this will dissolve the fossil structure.
Do not use excessive force when sanding—Petoskey Stones are soft and can wear away quickly.

With time and attention, properly polished Petoskey Stones will display their natural beauty far better than any artificial coating could provide.

Petoskey Stones have a fascinating history, from their origins as ancient corals to their journey through glacial transport. Whether you’re a collector, a geologist, or a curious explorer, these fossils offer a tangible connection to Earth’s distant past.

Before Vinegar

After a Week in Vinegar

Petoskey Stone in Polyurethane

Conclusion

Petoskey Stones are more than a beautiful fossilized coral—they are a tangible link to Michigan’s geological past and a reminder of the Devonian seas that once covered the region. Whether you find one along a rocky beach, gravel road, or limestone quarry, each stone holds a unique story of time, transformation, and discovery.

This guide comprehensively looks at what makes Petoskey Stones unique, from scientific classification and global distribution to collecting, identification, and polishing techniques. Whether you are a seasoned rockhound or a beginner collector, there is always more to learn and appreciate about these fascinating fossils.

If you have any questions, comments, or personal experiences with Petoskey Stones, please post them below or via the contact form.

The Besser Museum for Northeast Michigan in Alpena houses a massive boulder-sized Petoskey Stone, estimated to weigh around 4 tons!

Source: Detroit Free Press

Acknowledgments

I would like to thank the following for their assistance, either in providing knowledge, or selling me books/rocks for this presentation.

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Last Updated on 17 January 2025 by Angel Doran