I gave a pared-down version of this presentation on the Petoskey Stone, Michigan’s state stone, to the Ozark Mountain Gem & Mineral Society on 28 Feb 2022. While researching it, I found that much of the information on the internet (and sometimes books and magazines) is incorrect, so I spent about four months working through as much as I could find to collate the best available into one location.
Petoskey Stones
© Angel Doran
What is a Petoskey Stone?
Fossilized Colonial Rugose Coral
Corals are invertebrate marine animals from the Ordovician to now (with a 14-million-year break during the Triassic).
Corals consist of a polyp (hollow cylindrical sac) attached to the seabed, with a mouth surrounded by tentacles. They do not have other organs or a central nervous system but extract calcium carbonate from seawater and excrete it into an external skeleton for protection.
Rugose (wrinkled) corals existed from the Ordovician to the end of the Permian. Individual colonial corallites remained small, though colonies could get up to several feet across.
Rugose corals probably lived similarly to modern reef-forming stony corals, using their tentacles to sting and capture prey. Scientists are divided on whether rugose corals had a symbiotic relationship with algae because they were a small part of reefs, unlike tabulate corals which did the primary reef-building.
This symbiotic relationship helped the coral by the algae using the coral’s waste products for photosynthesis, which the algae used for its food. Byproducts of photosynthesis include oxygen and carbohydrates, which the coral needs.
Corals are extremely temperature sensitive, not because they can’t tolerate cold water, but because the algae can’t survive outside of 73° – 84° F. Water must also be clear to use the sun for photosynthesis. Some corals can live in greater, colder depths but adapt to life without algae. This isn’t something that happens rapidly, however.
Coral Terminology
- Polyp: single coral animal consisting of a soft body with mouth, tentacles, and gullet; this portion of the coral is not visible in the fossil record
- Corallite: the skeletal cup in which the polyp sat and could retract itself into; secreted by the polyp
- Calyx, calice (pl. calyxes, calices): the middle of the corallite where the polyp resided
- Columella: center of the calyx where the septa intertwine or form a dome-shaped or pillar-like projection
- Septum (pl. septa): radiating support plates from the calyx
- Tabula (pl. tabulae): as the polyp aged, it continued to deposit calcium carbonate in the corallite, raising the walls; as it became submerged in sediment, it built floors underneath it to raise itself up
- Dissepiment: curved support plates connected to septae and tabulae that provided vertical stability
- Carina (pl. carinae): vertical bar on septum
Devonian Period
During the Devonian, North America, Greenland, and Europe were consolidated in one massive supercontinent along the equator, called Laurussia, and surrounded by warm, shallow seas. The lower peninsula of Michigan was completely covered by these shallow waters.
Terrestrial life evolved from bacteria and algal mats (few cm tall) to the first trees (10-meters tall) and the first tetrapods and terrestrial arthropods appeared.
Marine life included rapid diversification in fish from jawless, armored bottom-dwellers to the first lobe-finned fish. Corals and sponges began forming the first extensive reef systems. Reefs protect coastlines from 97% of wave action, preventing erosion, and providing a protective environment for numerous species.
Location of Michigan during the Middle Devonian
Source: Petoskey Stone, Finding, Identifying, and Collecting Michigan’s Most Storied Fossil by Dan R. Lynch
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
Source: University of Michigan Museum of Paleontology
Devonian Mass Extinctions
Mass extinctions are a part of life. There have been numerous over the course of earth’s history, but five have been the most deadly. These five mark the end dates for the Ordovician, Devonian, Permian, Triassic, and Cretaceous.
Changes in atmospheric chemistry and climate caused all five of these mass extinctions in different ways. For the Devonian, a series of mass extinctions throughout the period eliminated 70-80% of all animal species. This is actually the smallest of the five and primarily affected marine life. One of these killed of nearly all coral between the Middle and Upper Devonian.
Geologists have noticed Devonian strata includes a layer of black shale that is characteristic of low oxygen environments. The current theory to how this happened is the development of terrestrial deep-rooted plants contributed to silicate weathering and soil production.
As runoff moves nutrients and carbonates from the soil and rocks into rivers and seas, the carbonates get trapped in sediment instead of returning carbon to the atmosphere. This lowers the amount of carbon dioxide, lowering global temperatures.
The extra nutrients encourage marine plant growth, primarily quick-growing algae, called algal blooms. The algae block sunlight from the lower levels and when they decompose, oxygen-utilizing bacteria feed on the decay. This lowers oxygen levels within the water.
The combination of lower temperatures and lack of access to sunlight, stressed the relationship between the coral and algae, causing the coral to kick the symbiotic algae out of its cells (called bleaching because the algae gives the coral its color). While coral can adapt to living in colder temperatures and feeding itself, the lower levels of oxygen didn’t give them the time needed to adapt.
How Coral Turns into Limestone
When coral dies, it becomes buried in sediment on the seafloor. As the polyp decays, the calyx fills with sediment. Additional layers of sediment building on top, along with calcium carbonate trickling down, compact and cement the coral skeletons, other marine skeletons, and sediment and turn it 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
Earth’s outermost layer, the crust, is broken up into 15 to 20 tectonic plates. They lie on top of the partially molten mantle. Due to radioactive processes in the earth’s interior, the plates move from two to 15 centimeters per year. When the plates slide against each other, they cause earthquakes. When they collide, one will dip under the other creating mountains.
Geologists have mapped out how they think these tectonic plates have moved over hundreds of millions of years by using paleomagnetic data to measure where rocks where located and comparing rocks in different locations. When the same type of rock is found on two continents, they can determine what timeframe those continents were close together. Also, they look like they fit together like a puzzle.
From the middle Devonian to present, Michigan moved from its location south of the equator to between the 41st and 49th parallels. (The 45th parallel, or the midway point between the equator and the North Pole, runs a little further north than the halfway point of northern Michigan.)
By the late Carboniferous, seas has retreated enough that Michigan was above water and was never submerged again. Michigan went from swampland to a semi-tropical jungle. Trees in the watery portions of the jungle were submerged and eventually turned into coal.
During the Permian, all of the land masses come together into one supercontinent, called Pangea. Michigan was far inland. This is one of the reasons there are no fossils (except a few spores) from the Permian to the Quaternary periods. Fossils rarely form on land because of erosion.
Map of the Tectonic Plates
Source: USGS
Pangea Map
Source: Wikipedia
45th Parallel through Michigan
Adapted from: Google Maps
Quaternary Ice Coverage
During the Quaternary, Michigan has been completely covered by ice during all four of the major North American glaciations. As ice encroached on land, it glided over the top picking up debris. The debris scoured the landscape the same way as using sandpaper on a rock. When the ice retreated, it left this debris in the form of glacial till behind.
As the ice advanced and retreated, it carved up the landscape creating lakes and grinding down the bedrock. Topping this bedrock is a layer of glacial till that is thin along the coastlines and much thicker in the interior.
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.
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.
Scientific Classification
Scientific classifications change as scientists learn more and reassign plants and animals to different taxonomies. The Petoskey Stone is no exception.
- 1866, Newton Horace Winchell – Acervularia davidsoni (based on Henri Milne-Edwards and Jules Haime in 1855)
- 1876, Carl Ludwig Rominger – Cyathophyllum davidsoni (Acervularia has a “central portion of the polyp cells surrounded by an internal wall,” but the Michigan ones did not)
- 1935, William Dickson Lang and Stanley Smith – Cyathophyllum davidsoni to Prismatophyllum davidsoni (established Prismatophyllum and provided diagnostic features that allowed for Michigan specimens to place in that genus)
- 1939, Laurence L. Sloss – Prismatophyllum percarinatum (P. davidsoni from France do not have carinae)
- 1970, Erwin C. Stumm – Hexagonaria percarinata
Michigan Hexagonaria Species
Erwin C. Stumm, was the Professor of Geology and Curator of Paleozoic Invertebrates at the University of Michigan. He was widely considered a leading expert in the world on Devonian invertebrate fossils. Deep Blue Documents (University of Michigan) has 56 of his papers and his name is mentioned in another 5,947.
His Corals of the Traverse Group of Michigan, a 13-part series he and others wrote from 1949-1969, is an in-depth look at Devonian coral fauna found in northern Michigan. Unfortunately, he passed away before he was able to finish the entire series. Part 13, Hexagonaria was the last he completed, just 15 days before he died.
In it, he described nine separate Hexagonaria species from specimens Sloss and G.M. Ehlers had previously collected. These are the nine species and the formations each were found.
- Hexagonaria alpenensis: Alpena Limestone
- Hexagonaria anna: Bell Shale into upper part of the 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
He also provided a chart of eight characteristics that can be used to differentiate the species.
Stratigraphy
Stratigraphy is the branch of geology concerned with the order and relative position of strata and their geological time scale. Geologists sequence rock strata into mappable units determining what timeframe the rocks are from.
The lower peninsula of Michigan is centered on a basin, appropriately called the Michigan Basin. It is deepest in the central part of the state and curves upward along the coasts. This is why older strata are closer to the surface along the coasts than along the interior of the state.
Michigan Hexagonaria are only found in situ in the Traverse Group and Bell Shale in northern Michigan. This corresponds to the Middle Devonian. While there are other Middle Devonian strata in Michigan and Traverse Group near Detroit, Hexagonaria are only found up north.
The Traverse Group is broken out into more than 20 formations separated into three geographic areas, Northwestern, North Central, and Northeastern. Northwestern covers Emmet and Charlevoix Counties, North Central is Afton to Onaway, and Northeastern covers Presque Isle and Alpena Counties.
Michigan Basin
Source: State of Michigan
Traverse Group Locations
Source: Devonian Strata of Northern Michigan
Northwest Michigan
Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties
North Central Michigan
Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties
Northeast Michigan
Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties
Hexagonaria alpenensis
- Alpena Limestone (northeast lower peninsula Michigan)
- White, light gray, or light brown, containing many bioherms; extremely variable composition
- Smallest species
Alpena Limestone
Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties
Hexagonaria anna
- Bell Shale, Rockport Quarry Limestone, and Ferron Point Formation (northeast lower peninsula Michigan)
- BS – Clay shale, soft, readily disintegrating, gray-blue, fossiliferous
- RQL – Limestone, gray to brown, sublithographic, specks of yellow calcite
- FPF – Clay, greenish-gray forming a densely packed surface
- Also found near Antwerp, Ohio
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 attenuata
- Alpena Limestone, Four Mile Dam Formation (northeast lower peninsula Michigan)
- AL – White, light gray, or light brown, containing many bioherms; extremely variable composition
- FMD – Limestone, brownish gray, with sandy shale partings
- Charlevoix Limestone (northwest lower peninsula Michigan)
- Kegomic Quarry Syncline – Eastern limit of Emmet County and the valley of Bear Creek, exposes the Petoskey Formation at Kegomic Quarry north of the line through the outcrop of the underlying Charlevoix Limestone at Bay View
- Very similar to H. subcarinata; longer major septa, lack of distinct boundary between dissepimentarium and tabularium in transverse, corallites somewhat smaller
- Only species found on both coasts
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 cristata
- Gravel Point Formation (northwest lower peninsula Michigan)
- Mainly from the 10-15ft limestone interval between the lower and upper blue shales, Little Traverse Bay
- Largest species
Gravel Point Formation
Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties
Hexagonaria fusiformis
- Genshaw Formation, Ferron Point Formation, Newton Creek Limestone (northeast lower peninsula Michigan)
- GF – Limestone, gray or light brown; small bioherms in places
- FPF – Clay, greenish-gray forming a densely packed surface
- NCL – Limestone, dark brown, crystalline
- Similar to H. anna; has longer, rhopaloid major septa and 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 potterensis
- Potter Farm Formation & Thunder Bay Limestone (northeast lower peninsula Michigan)
- PFF – Limestone, brownish gray, with sandy shale partings
- TBL – Limestone, bluish, argillaceous, weathering to rusty brown; exposed at water level or just below at Partridge Point
- Calyx walls intermediate between H. percarinata and H. 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 profunda
- Petoskey Limestone, Kegomic Quarry (northwest lower peninsula Michigan)
- PL – Gray, shalelike limestone
- Only one found, identical to Cedar Valley Limestone, Iowa
- Unlikely to see this one in Michigan
Petoskey Limestone
Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties
Hexagonaria subcarinata
- Alpena Limestone (northeast lower peninsula Michigan)
- White, light gray, or light brown, containing many bioherms; extremely variable composition
- Very similar to H. percarinata, except lack of abundant carinae
- Less likely to be found with shale
Alpena Limestone
Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties
Hexagonaria percarinata
- Gravel Point Formation, Little Traverse Bay (northwest lower peninsula Michigan)
- Limestone, brownish gray, with sandy shale
- Most common species
- Can be distinguished from other species by false inner wall created by crowding of dissepiments and carinae at the axial ends of the minor septa and in being much more heavily carinate
Gravel Point Formation
Adapted from: Devonian Strata of Emmet & Charlevoix Counties, Afton to Onaway, and Alpena and Presque Isle Counties
Gravel Point Formation
These are images of the Gravel Point Formation behind Bay Harbor in Petoskey, Michigan.
Worldwide Hexagonaria
Worldwide, over 70 Hexagonaria species have been identified in 31 different locations. (It is entirely possible some of these have been misidentified and changed taxonomy).
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
Afghanistan
Alaska
Alberta
Arizona
Australia
Belgium
British Colombia
China
France
Germany
Indiana
Iowa
Iran
Kazakhstan
Kentucky
Manitoba
Michigan
Morocco
New Mexico
New York
New Zealand
Northwest Territories
Nunavut
Ohio
Ontario
Poland
Russian Federation
Spain
Tajikistan
United Kingdom
Vietnam
Yukon
Worldwide Hexagonaria Locations
Adapted from: Google Maps
Where to Look in Michigan
All of the previous information about where to find Hexagonaria in situ is correct, but people rarely take the time to dig them out of the ground. It’s much easier to go to a lower peninsula rocky beach, find them scattered in gravel throughout the state, or look through glacial moraines.
People have found Petoskey Stones at pretty much every lower peninsula rocky beach in the state. The best places to look are around Little Traverse Bay, but Grand Traverse Bay, Leelanau Peninsula, and Roger’s City to Alpena are also great locations. The reason for this is the Great Lakes didn’t exist when the coral formed. The formations that contain these fossils continue out into the lakes, primarily in the shallower bays. Waves bring the rocks into shore all the time, though the best time to look is in the spring after breakup or after a storm.
NOTE: There is a 25-lb weight limit per year on taking rocks from any State land in Michigan, including State parks and Lakes. The National Parks (Sleeping Bear Dunes, Pictured Rocks, and Isle Royale) are off-limits to rock collecting.
Northern Michigan was the one of the largest producers of limestone in the world and still has the single largest limestone quarry in the world (Calcite in Roger’s City). In 1978, Michigan had 31 limestone quarries in northern Michigan and the eastern half of the U.P. Limestone has a variety of uses including road and building constructions and for making cement. Northern Michigan has a lot of gravel roads, which were primarily supplied with local stone.
When the glaciers receded, Devonian fossils ended up all over the state in glacial till.
Origins of the Name
Legend says the Petoskey Stone was named after a great Ottawa chief, Pet-O-Sega, whose name meant “Rising Sun,” “Rays of Dawn,” or “Sunbeams of Promise.”
It more likely came about because it’s found in the Petoskey area and people were selling them as souvenirs, though it’s possible someone was savvy enough in marketing to associate the name with the stone. I find it unlikely that whomever first named it did so completely independent of any knowledge of how the city was named.
Petoskey was named after Biidassige (Light that is Coming), an Odawa fur trader and businessman born in 1787. His anglicized name was Ignatius Petoskey (also Neyas Petosega).
Bear River/Creek (Mukwa Ziibing) was 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, Michigan Native Americans were purchasing back land they had just sold to the Federal Government in the Treaty of Washington (1836). The Treaty ensured they would have permanent hunting and fishing rights, education, money, and services, but Congress altered the terms of the treaty to only five years. The Native Americans thought that by purchasing back the land, they would have the same rights as whites and could not be forcibly removed to west of the Mississippi River.
In 1852, a Presbyterian Mission was established in Bear River when a minister arrived with a deed to 80 acres of the 440. Warren Petoskey, the great-great-grandson of Ignatius Petoskey, wrote in his book, Dancing the Dream, that when Michigan became a State, Petoskey did not know he was required to pay taxes on the land. The State sold the land to the Presbyterian Church for back taxes. Petoskey did not contest the loss because he was afraid to draw the attention of the government and potentially have the Native Americans forcibly removed to Kansas.
In 1855, the Odawa entered into another treaty with the U.S. Government, The Treaty of Detroit (1855). This time, the Native Americans would be given 40-80-acre plots of land within specific townships. These areas were reserved for the Natives for 10-years and they could not sell or transfer the allotment. Exemptions were made for missions, churches, schools, and settlers already living in the areas. After the 10-years were up, Michigan could begin selling the leftover land and the titles were issued to the Native landowners.
According to Harriet Kilborn in an essay she wrote on The History of Petoskey Area, Petoskey and his sons added to the 360-acres they still had with allotted plots from the Treaty of 1855 and by the time the land was open for purchase, they owned most of downtown Bear River.
In 1873, the Grand Rapids and Indiana Railroad added a stop in Bear River. This brought an influx of people to the area. Dr. William Little applied for a post office and the job of postmaster, and asked for the town to be renamed Petoskey in Ignatius Petoskey’s honor.
Petoskey and his sons lost most of the lands. He died in 1885 and still has descendants living in the area today. His granddaughter, Ella Jane Petoskey, was a signatory to Act 89 of 1965 naming the Petoskey Stone the state stone of Michigan.
So, What is a Petoskey Stone?
Petoskey Stone is a colloquial, nickname, or marketing name for Hexagonaria found in Michigan.
Some people insist it must be found in northern Michigan, but glaciers dragged them all over the state (and Midwest) and the plethora of limestone quarries provided ample gravel to fill all of the gravel roads in the state.
Others insist only H. percarinata is a true Petoskey Stone. As far as I can tell, Laurence Sloss first identified which species of Hexagonaria is known as the Petoskey Stone in his doctoral dissertation to the University of Chicago in 1937 when he wrote “Prismatophyllum percarinatum is the coral of the familiar ‘Petoskey stone’ of northern Michigan.” It’s obvious from what he wrote that Petoskey Stones were already a well-known souvenir at least through the Great Lakes region since Sloss was from California and attending school in Illinois.
I don’t know whether or not Petoskey Stones were being collected and sold outside of Petoskey in 1937 or whether or not all of the Hexagonaria species were being sold as Petoskey Stones at the time. But, they are now. Souvenir shops throughout the state sell Petoskey Stones. People collect them from both Lake Michigan and Lake Huron (or wherever they happen to find them in the state) and sell them online as Petoskey Stones. Most people don’t know there are multiple species. If they do, they probably don’t know how to tell the difference between them or they don’t have the scientific equipment necessary to prepare and examine them.
Plus, most of the ones you find on the Lakes are coming from bedrock under the water, not the formations listed above. As far as I can tell, there has not been a geological undertaking to determine which species of Hexagonaria are in the Lake bedrock. Nor has there been a thorough examination of rocks found on different beaches to determine their species.
I’ve seen people argue online about whether a rock is a “true” Petoskey Stone. They state you can only find it in the Gravel Point Formation and then mention their “true” one was found on a beach around Leelanau. The Gravel Point Formation is not exposed in Leelanau and Erwin Stumm was very specific in his Corals of the Traverse Group of Michigan, Part 13 that you can only find H. percarinata in the Gravel Point Formation along Little Traverse Bay (emphasis is mine).
I’ve had someone tell me you can’t even find Petoskey Stones in Petoskey, they are just called that because the “Rising Sun” meaning behind the name works so well for the coral. They were irrationally dedicated to their belief that only “true” Petoskey Stones are ones found in their area.
I’ve heard others proclaim that they have an H. percarinata and it’s the “true” Petoskey Stone. When I asked them how they know it’s an H. percarinata and not an H. subcarinata, they had no idea how to tell me the difference.
I think any Hexagonaria found in the Great Lakes region could be called a Petoskey Stone. Let’s stop fighting among ourselves over what is a “true” Petoskey Stone. Leave the species designations to the paleontologists. Only call it an H. percarinata if you have examined a slice of it with a microscope, or pulled it straight out of exposed Gravel Point Formation along Little Traverse Bay, and can honestly tell what species of coral it is. Otherwise, it’s a Petoskey Stone.
Visual Identification
Limestone is normally white, but Petoskey Stones took on gray and brown tones from crude oil in their environment. The “eyes” filled with mud and silt, making them darker than the corallite skeletons.
Ones found along the coasts are usually smooth and contoured like a bar of soap. Most Petoskey Stones are pebble to fist-sized, with egg-sized being average. When dry, it looks like regular limestone, but when wet, the hexagon pattern starts to stand out.
Petoskey Stones found in the interior tend not to be as worn down. You can see the coral structure much better. Sometimes you will even find ones with the stem still attached, called a mushroom cap.
Occasionally you will find the tabula structure on the “top” of the stone as if the coral grew sideways or it will be mixed with other fossils.
Dry, Unpolished Petoskey Stone
© Angel Doran
Wet, Unpolished Petoskey Stone
© Angel Doran
Smooth, Unpolished Petoskey Stones
© Angel Doran
Rough, Unpolished Petoskey Stones
© Angel Doran
"Mushroom Caps"
© Angel Doran
Petoskey Stones with Sides on Top
© Angel Doran
Petoskey Stones Mixed with Other Fossils
© Angel Doran
Color Variations
Petoskey Stones can also be a variety of colors depending on what was present when it formed. The only way to know for sure is to use x-ray fluorescence, but the leading theory is that iron causes most of it.
- Ferric oxide (Fe2O3) = red, orange, or yellow (hematite, limonite)
- Ferrosoferric (Fe3O4) oxide = blue (magnetite)
- Mixture of ferrosoferric oxide with large amount of ferric oxide = purple
- Mixture of ferrosoferric oxide with small amount of ferric oxide = green
- Ferrous oxide (FeO) = colorless, will dilute color when mixed with other types (wüstite)
Other potential causes could be copper (blue-green), nickel (blue), low-oxygen sediments (blue-gray), or manganese (purple).
You will often see coral in other locations that was fossilized through silicification. This is where silica minerals replace the calcium carbonate in the skeleton. Silica better preserves the coral structure and allows for easy extraction of the fossil from other materials because silica is much harder than calcium carbonate and doesn’t dissolve in acids.
Petoskey Stones are rarely silicified. When they are, it happens after the original fossilization and the silica obscures rather than enhances the coral structure.
Brown Petoskey Stones
© Angel Doran
Gray Petoskey Stones
© Angel Doran
Multicolored Petoskey Stones
© Angel Doran
Silicified Petoskey Stones
© Angel Doran
How to Spot a Fake
Petoskey Stone is a well-known marketing name for fossilized coral. You will find that people often sell their coral fossil as Petoskey Stone when it is not Hexagonaria from northern Michigan. While dishonest (or uninformed), that doesn’t mean the coral fossil is not a real fossil or that it isn’t as pretty, or good, or interesting as a Petoskey Stone. It just means it’s not Hexagonaria from northern Michigan.
Please see this guide on how to tell the difference between Petoskey Stones and other types of Devonian coral found in Michigan and other types of Rugose corals.
Coral fossils from Morocco are frequently misidentified as Petoskey Stones. While there has been a recorded find of Hexagonaria in Aferdoiu el Mrakib, Morocco, I have not been able to find an image of one.
One of the most frequent Moroccan coral fossils sold is Lonsdaleia. The way you can tell the difference between Lonsdaleia and Hexagonaria is the septa on Lonsdaleia do not go all the way to the corallite walls. Instead they stop at the edge of the calyx and there is a large zone of bubbly dissepiments between the calyx and corallite walls.
Moroccan Lonsdaleia
© Angel Doran
Acrocyathus is very similar to Lonsdaleia, except it has the lens-like columella. Acrocyathus can be found in the Mississippian Bayport Limestone in Michigan.
Acrocyathus
© Angel Doran
Acrocyathus
© Angel Doran
Indonesian agatized coral is a type of silicified Scleractinia. It is normally pink or yellow and is sometimes heated to produce more vivid colors. It’s also known as Chrysanthemum Coral. Of the two I have here, neither were marketed as a Petoskey Stone.
These last items are not stone at all. These are “Petoskey Stones” made out of polymer clay and are marketed that way. I just thought they were truly amazing and wanted to show them off to others.
Indonesian Agatized Coral
© Angel Doran
Petoskey Stones in Polymer Clay
© Angel Doran
How to Clean and Polish
You can clean Petoskey Stones with a soft-bristled toothbrush and vinegar. Be careful! Too much time in vinegar can destroy the fossil. Be sure to wash all of the vinegar off or use baking soda to neutralize it.
Petoskey Stones sand very easily, but they are very porous and often have spots that are difficult to polish. Sometimes you can work past it, sometimes, it’s just best to live with the defect.
I use a grinder to smooth out cracks and divots, then hand sand with 60-10,000 grit sandpaper. Spend a lot of time on the coarser grits and you will start seeing a shine by 3,000 grit at the latest. If not, go back down a few steps and start over. Doing this will not require soaking them or baking them in mineral oil.
You can tumble Petoskey Stones, but I have not gotten good results with it. I like to tumble them for a stage or two and then switch to hand sanding. This saves me the hard work of the coarser grits.
Here are some suggestions on how to tumble Petoskey Stones with notes from me:
- Start with 320 or 500 silicon carbide & a thickening agent to make a slurry (syrup, sugar, or molasses) – I use guar gum because I don’t want to deal with the sticky mess
- Use 600 or 800 silicon carbide with a ratio of 1 / 1 or 2 / 1 media (walnut shells or corn cob) to Petoskey Stones – I use cut up neoprene foam
- If you used 600 silicon carbide, follow with 600 aluminum oxide and media; otherwise jump to polishing stage
- Use cerium oxide or 0.5-0.8 micron aluminum oxide mixed with media and thickener
Please don’t coat them with epoxy, spray them with polyurethane or paint them with nail polish (unless it’s part of an art project). With a little time and attention, they can look so much better.
Before Vinegar
© Angel Doran
After a Week in Vinegar
© Angel Doran
Petoskey Stone in Polyurethane
© Angel Doran
Conclusion
If you have any questions or comments, post them below or send me a message through the contact form.
Acknowledgments
I would like to thank the following for their assistance, either in providing knowledge, or selling me books/rocks for this presentation.
- 2020mountionc-27
- Ancient Michigan
- Anybook Ltd
- Asa Asa, Missouri Fossil Hunters
- BackToOurRootsCo
- beesal
- Better World Books
- bookintime
- brandoshilt
- broken_and_branded
- callistodesigns
- Camp Retriever
- Crystalman’s
- dinomite-rocks
- Down2Earth66
- Fossil Age Minerals
- Fossil Era
- FossilsByJosef
- gjo.uk.rhsoha
- GrandDadRocks
- GrauppsCabochons
- JonesMineralCabinet
- KraftByKara
- LakeshoreStonesLLC
- LittleLinkShop
- Little Traverse Bay Bands of Odawa Indians
- Mark Hettich
- MichiganGem
- michigan-rocks
- midwestshoresco
- MorianMiner
- Ozark Mountain Gem & Mineral Society
- The Fossil Forum
- PlanetGems
- The Polkadots
- Prettyfossilsnthings
- pr0teusunbound
- QuartzQueenTreasures
- RockCityByWill
- RocksforSocks
- Rocky Mountain Textbooks
- SaltyPetuniaVoyages
- SegerRockShop
- selltotheworld
- shh!actnatural
- Silver Trees Books
- spencer58
- SpottedWoodchuck
- SteveCabbingDesigns
- ukge-silvergeo
- VintagePennyLane
- Whitehorsewoodworks
References
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Last Updated on 10 December 2023 by Angel Doran