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Petoskey Stone

Michigan State Stone

Polished petoskey stone; photo by cobalt123 on Flickr (noncommercial use permitted with attribution / share alike).

Official State Stone of Michigan

Michigan designated petoskey stone as the official state stone in 1965. Petoskey stone (Hexagonaria pericarnata) is actually not a stone but fossilized coral. All State Rocks & Minerals - All State Fossils

In prehistoric times (about 350 million years ago), Michigan was covered by a shallow sea that teemed with coral colonies.