These polished Petoskey stones can range from 3/4" to 1.5" across. Photos are for representation only. Each piece is unique. Petoskey stones are a variety of Calcite. They were 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 Travers City to Alpena. The honeycomb patterned stone has a hardness from 4 to 5 and is the official Michigan State Stone.