“PETOSKEY STONE” Michigan's State Stone
Rock&Gem Magazine|September 2021
FOSSIL FINDS
JOSEPH "PALEO JOE" KCHODL

The Hexagonaria coral(actually Hexagonaria percarinata) is commonly called "Petoskey Stone"

To paleontologists, this coral truly does not become a “Petoskey Stone” until someone slices and polishes it (and charges people lots of money for it). Then it becomes a Petoskey Stone. In its rough natural form, it is really a piece of Hexagonaria coral. Walking along the northern Lower Peninsula shorelines of lakes Huron and Michigan, it is easy to see this fossil’s natural attraction on people. Looking into the water, one can sometimes see this coral, rounded by relentless waves, worn smooth showing its beautiful internal structure. It is easy to see that rockhounds, jewelry makers and almost everyone would be taken by this attractive fossil. Once removed from the water, however, it quickly dries and loses its bright polished shine. To achieve that permanent shine it is necessary to polish the rock by grinding and sanding it with various grits of sandpaper and abrasives, finally finishing it with a polishing compound to bring out the beautiful luster of the stone. Then it is a Petoskey Stone. There are at least nine species of Hexagonaria, but the only true “Petoskey Stone” is the H. percarinata.

Corals are marine organisms that are made up of many – sometimes thousands of hard calcium carbonate exoskeletons called corallites. Each corallite contains a polyp – an individual multi-cellular animal. There are two major types of corals. Solitary corals growing by themselves, and colonial corals, growing in a tight community of genetically identical polyps. The polyp is the actual living individual creature that inhabits each corallite. As the coral grows, it extends the calcium carbonate exoskeleton and seals off part of the base.

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