Petoskey is a type of fossilized colony coral found only in Michigan.Michigan’s land mass is part of the early continents. Around 1 billion years ago, the tectonic plate Michigan rests upon began to crack. The tectonic plate was prevented from breaking when it bumped against a neighboring plate. The mid-continent rift pushed Michigan’s land mass underwater.
Under the sea, and near the equator, Michigan witnessed the Cambrian explosion, when multicellular life began to arise in mass. Over time, a giant coral reef formed in this sea. Then the Late Devonian mass extinction, ~ 350 million years ago, put an end to these reefs.
Around 1 million years ago, Michigan was about as north as it is now & was covered by ice - the only state completely covered by glaciers during the ice age.
The glaciers ebbed & flowed across the land, & finally receded ~12,000 years ago, pushing millions of years of sediment around. This exposed the fossils of the coral reefs that had died off, in the form of Petoskey stones. I have found em in Ontario on Huron