What Is a Petoskey Stone?
A Petoskey stone is a fossilized coral (Hexagonaria percarinata) from the Devonian period, about 350 million years ago. The hexagonal pattern of individual coral polyp chambers is clearly visible when the stone is wet or polished. Michigan was covered by a shallow tropical sea during the Devonian, and these coral colonies were preserved in the limestone bedrock.
Where to Find Them
Petoskey stones are found along the shorelines of Lake Michigan and Lake Huron, especially near Petoskey, Charlevoix, Traverse City, and the Sleeping Bear Dunes area. The best hunting is after storms that churn up new material from the lake bottom. Late fall and early spring produce the most exposed stones on gravel beaches.
How to Identify Them in the Field
Look for gray to tan oval stones with a faint hexagonal pattern on the surface. The pattern is often difficult to see on dry stones. Wet the stone with lake water or saliva — the hexagonal coral chambers immediately become visible. Size ranges from marble-sized to football-sized, though large ones are increasingly rare on popular beaches.
How to Polish a Petoskey Stone
Start with 220-grit wet/dry sandpaper, then 400, 600, 1000, and finally 2000-grit. Keep the stone and paper wet throughout. Finish with a polishing compound (cerium oxide or automotive polishing compound) on a cloth. The final result reveals stunning detail of the coral chambers. Alternatively, a rock tumbler works but requires a longer cycle.
Other Michigan Gems Worth Finding
Michigan's Upper Peninsula also yields chlorastrolite (Isle Royale greenstone — state gem), thomsonite, copper nuggets, and prehnite in the Lake Superior basalts. The Keweenaw Peninsula has native copper in float boulders. Houghton County has world-class copper mineral specimens.