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.