A Petoskey stone is a rock that is composed of a fossilized coral, Hexagonaria percarinata. The stones were formed as a result of glaciation, in which sheets of ice plucked stones from the bedrock, depositing them in the north part of Michigan's lower peninsula.
Petroskey stones are found in the Gravel Point Formation of the Traverse Group and were originally deposited during the Devonian period. When dry the stone resembles ordinary limestone but when wet or polished using lapidary techniques, the distinctive mottled pattern of the fossil emerges. It is sometimes made into decorative objects. Other forms of fossilized coral are also found in the same location. The name comes from an Ottawa Indian Chief, Chief Pet-O-Sega; a town, Petoskey, Michigan, is also named after him, and is the center of the area where the stones are found. The stones are commonly found on the beaches and in sand dunes.
In 1965, it was named the state rock of Michigan.
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