The size ranges from 1.75" to 2.5" diameter. Petoskey Stones are formed by the fossilization of ancient coral, Hexagonaria. The Petoskey stones are named after the town of Petoskey Michigan. Petoskey Stones are found on many beaches of Lake Michigan and Lake Huron, as well as in rock quarries from Traverse City to Alpena. Fossilized coral is common in the limestone around the shores of Lake Michigan. The coral was originally deposited during the Devonian period, about 350 million years ago. The honeycomb patterned stone has a hardness of 4 to 5 on the Moh's Hardness scale and is the official Michigan state stone.