MINERAL OF THE MONTH: September 2011 – Petoskey Stones

In 1965 the state stone for Michigan was designated as the Petoskey stone. Petoskey stones are fossils of coral colonies called Hexagonaria. “Hex” comes from hexagon, because the coral is usually a six-sided polygon. The coral grew 350 million years ago, 150 million years before the dinosaurs! It grew only in shallow tropical salt-water seas where lower Michigan is now. The coral stacked in layers, then fossilized into rock.

The Petoskey stones found on the beaches of Lakes Michigan and Huron were formed as a result of glaciation, in which sheets of ice plucked stones from the bedrock, grinding off their rough edges and depositing them primarily in the northern portion of Michigan’s Lower Peninsula. In some areas of Michigan, complete fossilized coral colony heads can be found. The formations and specimens found inland tend to be rougher since they have not been weathered as much by the wind, water, and sand from the shoreline. The movement of the frozen lake ice acting on the shore during the winters is thought to turn over stones at the shore exposing new Petoskey stones at the water’s edge each spring.

Similar fossil corals occur in a variety of locations; however the name Petoskey stone should only be applied to those from Michigan which exhibit a six-walled coral structure that has the distinctive “eye” pattern within each cell.

When ancient glaciers pressed down on the center of the Lower Peninsula, the layer of fossilized coral rose up to form a ridge and created the dish shaped “Michigan basin”. The exposed layer of rock is where Petoskey stones come from. Petoskey stones are found in the Gravel Point Formation of the Traverse Group. They are fragments of a coral reef that was originally deposited during the Devonian period. When dry, the stone resembles ordinary limestone but when wet or polished using lapidary techniques, the distinctive mottled pattern of the six-sided coral fossils emerges.

It is sometimes made into decorative objects. Other forms of fossilized coral are also found in the same location including the closely related favorites. Favosites is an extinct genus of coral characterized by polygonal closely-packed corallites (giving it the common name “honeycomb coral”). The walls between corallites are pierced by pores which allowed transfer of nutrients between polyps.

The name comes from an Ottawa Indian Chief, Chief Petosegay. The city of Petoskey, Michigan, is also named after him, and is the center of the area where the stones are found. According to legend, Petosegay was the child of a descendant of French nobleman and fur trader, Antoine Carre and an Ottawa princess. Petosegay, meaning “rising sun”, “rays of dawn” or “sunbeams of promise”, was named after the rays of sun that fell upon his newborn face. In keeping with his promising name, Petosegay was a wealthy fur trader who gained much land and acclaim for himself and his tribe. He was remarked upon to have a striking and appealing appearance, and spoke English very well. He married another Ottawa, and together they had two daughters and eight sons. In the summer of 1873, a few years before the chief’s passing, a city began on his land along Little Traverse Bay. The settlers christened the newborn city Petoskey, an anglicized form of Petosegay.


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