Topic: Gemology and Jewelry

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Petoskey stone

Geology Michigan Gemology and Jewelry/Gemstones subpage Gemology and Jewelry

A Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized rugose coral, Hexagonaria percarinata. Such 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 (and some in the northeastern) portion of Michigan's lower peninsula. In those same areas of Michigan, complete fossilized coral colony heads can be found in the source rocks for the Petoskey stones.

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. When dry, the stone resembles ordinary limestone but when wet or polished using lapidary techniques, the distinctive mottled pattern of the six-sided coral fossils emerges. It is sometimes made into decorative objects. Other forms of fossilized coral are also found in the same location.

In 1965, it was named the state stone of Michigan.

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