A Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized coral, Hexagonaria percarinata. They are fragments of a coral reef that was originally deposited during the Devonian period, about 350 million years ago. (Wikipedia) Photo by Mark Willocks.
Petoskey Stones or fossilized coral. / Pierres de Petoskey ou coraux fossilisés. / Resulting from glaciation and often polished to reveal the beautiful 6 sided pattern. / Formées par la glaciation et souvent polies pour révéler leurs magnifiques motifs hexagonaux. / Michigan. / USA.