Petoskey Stone is both a rock and a fossil. These finely detailed specimens contain large (and very visible) amounts of fossilized Rugose Coral — hence the hexagonal shapes you see throughout each stone.
The stones are each fragments of a long-extinct coral reef that existed during the Devonian period. Petoskey Stone is the state stone of Michigan. The stone (and a city in Michigan) was named after an Ottawa chief — Chief Pet-O-Sega.
All of the stones in this listing were recovered from shores near Charlevoix, Michigan.