Michigan State Rock
Despite its name, Petoskey stone is actually not a stone, but is both a rock and a fossil. It is composed of a fossilized coral calledHexagonaria percarinata. This rock is mainly composed of calcite, with quartz, pyrite, aragonite, silica, and other minerals sometimes also present.
Natural Petoskey stones have been rounded by water but their surfaces usually remain somewhat rough. When wet or polished, the stones’ characteristic pattern of six-sided coral fossils, as well as their color, is more visible. This detail has been preserved by the fossilization, or petrifaction, of the corals, in which the original living tissue was replaced by minerals from the sea.
A STATE SYMBOL
First discovered in Michigan, Petoskey stones have become an important geological symbol for the state’s rock collectors and tourists. Due to its impact on Michigan history, culture, and geology, Petoskey stone was named the state rock of Michigan in 1965.
Petoskey stone was named after an 18th century Ottawa Indian Chief, Chief Pet-O-Sega. Legend says that Chief Pet-O-Sega, possibly also Petosegay or Bedosegay, was a child of an Ottawa princess and a French fur trader with the John Jacob Astor Fur Company. His name means "rising sun" or "rays of dawn."
FORMATION AND OCCURRENCE
Michigan’s state rock was formed from ancient coral reefs that were alive in the Devonian period. The reefs thrived approximately 360 million years ago when warm, shallow, prehistoric seas covered what is now the Midwest United States. As the coral died, its organic matter was gradually replaced with minerals, or fossilized. The fossilization process took place over millennia. About two million years ago, Pleistocene glaciers moving across the area pulled the rocks from the bedrock and spread them around.
Petoskey stones are found almost exclusively on the beaches from Grand Traverse Bay to Little Traverse Bay and gravel deposits up to 30 miles inland on Michigan’s lower peninsula. They are particularly abundant in the tourist region of the same name, the area around the town of Petoskey. These rocks can also be found in Saint-Urbain-de-Charlevoix, Québec, Canada. Other members of this genus occur throughout the western and central United States and in Ontario, Canada.
Petoskey stones are easy to find and quite popular among amateur and seasoned collectors alike. Many Petoskey stones can be enhanced by lapidary techniques such as grinding, filing, polishing, and shaping. However, some stones that are too porous or have impurities would not benefit from such techniques.
Chemical Formula: CaCO3
Color: Occurs in all colors, sometimes even multicolored