Petoskey Stone This interesting stone is a variety of Calcite. It was formed by the fossilization of ancient coral, Hexagonaria. The coral lived in shallow waters and today's fossil often includes ancient silt and mud. Petoskey Stones are found on many beaches of Lake Michigan and Lake Huron, as well as in rock quarries from Traverse City to Alpena. The honeycomb patterned stone has a hardness from 4 to 5 and is the official Michigan State Stone.