During the Devonian Period, 350 million years ago, a shallow sea covered Michigan and much of what comprises the United States.  This land mass was located near the equator and a large coral reef covered much of Michigan.  This shallow sea was home to Bryozoans, Stromatoporoids, Trilobites, other marine animals, and numerous corals, including Hexagonaria percarinate.  The Hexagonaria percarinate's food was plankton.  The center of the coralite was its mouth tentacles were used for gathering food. 

Continental drift occurred and tectonic plates shifted.  The shallow sea dried up.

Siltification or mineralization occurred, and then, fossilization  of the dead organisms followed.  Calcite, silica, and other minerals have replaced the original coral structure.  Petoskey stone, one of these fossilized corals, is the coral most often found in the Petoskey, Michigan area.  Petoskey stone is a colony coral.  Each coralite in the colony is hexagon in shape, and has radiating lines inside, and thus looks like the rays of the sun.

Petoskey stones are a sedimentary rock.  This specific fossil is found only in the rock strata, Alpena Limestone, which is part of the Traverse Group of the Devonian Age.  

During the Pleistocene, some 1.6 million years ago, moving glaciers scoured Michigan's surface, took stone from the bedrock, and dragged Petoskey stone south toward Indiana and north toward Michigan's Upper Peninsula.  Found stones might be as small as your fingernail or as large as a basketball.  

Related fossil corals are also found in Iowa, Indiana, Illinois, Ohio, New York, Canada, Germany, and even Asia.  These corals are related but not identical.  The name Petoskey stone should not be used in conjunction with these other fossils, only those fossils found in Northwest Michigan.  

Petoskey stone is the remenant of an organism of a past geological age and is a great, modern day, vacation rememberance.