Petoskey stone << puh TOS kee >> is a patterned stone found only near Petoskey, Michigan. The stone is actually fossilized coral of the prehistoric species Hexagonaria pericarnata. The fossils come from limestone deposits formed about 350 million years ago when an inland ocean covered most of what is now Michigan.
The coral occurs in columnlike formations. When cut across, each column has a six-sided shape within it, accompanied by a pattern of lines that branches out from the center. This structure can be seen most clearly on a smooth, wet surface. Fossils of related corals can be found in Illinois, Iowa, New York, Ohio, Canada, and parts of Europe.
The natural action of water, or grinding and polishing by a craftworker, shapes the fossil into a gemstone prized by mineral collectors. Petoskey stone was designated as the official state stone of Michigan in 1965.
