Petoskey Stones are fossilized
colony corals (Hexagonaria percarinata). Approximately
350 million years ago warm salt water covered Michigan.
In the depths of this sea, the members of the coral
colony lived. It is a compound coral in that many
animals lived together in a colony. The soft living
tissue of corals is called the polyp. The polyp
hardens into corallites, a skeletal base that
secretes a limey substance, supports the polyp
and keeps it from being buried alive by bottom
debris. The limey skeletons were replaced by calcite
or silica in a cell-by-cell process called petrification.