From Wikipedia: A Petoskey stone is a rock and a fossil, often pebble-shaped, 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, grinding off their rough edges and depositing them in the northwestern portion of Michigan's lower peninsula. Petoskey stones are found in the Gravel Point Formation of the Traverse Group. They are fragments of a coral reef that was originally deposited during the Devonian period, about 350 million years ago. 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. In 1965, it was named the state stone of Michigan.