Unpolished, fossilized <i>Hexagonaria percarinata</i>, commonly known as <a href="https://en.wikipedia.org/wiki/Petoskey_stone" class="extiw" title="en:Petoskey stone">Petoskey stone</a>.  Sample found in the <a href="https://en.wikipedia.org/wiki/Sturgeon_River_(Cheboygan_County,_Michigan)" class="extiw" title="en:Sturgeon River (Cheboygan County, Michigan)">Sturgeon River</a>, <a href="https://en.wikipedia.org/wiki/Cheboygan_County,_Michigan" class="extiw" title="en:Cheboygan County, Michigan">Cheboygan County</a>.  Shown with <a href="https://en.wikipedia.org/wiki/Centimeter" class="extiw" title="en:Centimeter">cm</a> scale.
David J. Fred (<a href="//commons.wikimedia.org/wiki/User:Dfred" title="User:Dfred">Dfred</a>)
CC BY 3.0
<span class="int-own-work" lang="en">Own work</span>