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.