Petoskey stones are composed of fossilized skeletons of Hexagonaria percarinata, a type of coral from coral reefs that once covered all of what is now the state of Michigan, the USA, during the ancient Devonian period, some 350 million years ago. The stones were formed as a result of glaciation, in which sheets of ice scrapped the bedrock, picking up fragments, and then grinding off their rough edges and depositing them in the northwestern portion of Michigan's lower peninsula. Basically, Petoskey stones are just chunks of coral reef, and when dry the stone resembles ordinary limestone but when wet or polished, the distinctive mottled pattern of the six-sided coral fossils emerges. This prehistoric fossil is found across the state of Michigan along lakeshores and rivers in sediments commonly called the Traverse group. Since 1965, the Petoskey stone is Michigan’s official state stone.
Over 350 million years ago during the Devonian period, Michigan was quite different. Geographically, the region was located near the equator and covered by a warm, shallow, saltwater sea, where the colonial coral hexagonaria percarinata thrived with other marine life in tropical reefs. Then the earth’s plates moved and pushed Michigan north and above sea level. When glaciers came about two million years ago, the land was scraped and the fossils spread across the northern Lower Peninsula. The stone was named Petoskey because they are found in great abundance in the Petoskey area.
The name Petoskey comes from “Petosegay”, the son of an 18th century Ottawa chief, and it means “rays of dawn” or “sunbeams of promise.” The city of Petoskey was also named after the same person. Some say, the coral pattern in the stone looks like sun rays radiating from small suns.
Hexagonaria percarinata consists of tightly packed, six-sided corallites, which are the skeletons of the once-living organism. At the center of each corallite was the mouth, surrounded by tentacles that were used for gathering food and drawing the food into the mouth. This dark spot, or the eye of the corallite, has been filled with silt or mud that petrified after falling into the openings. Calcite, silica and other minerals have replaced the original soft tissues, called polyps, in each cell.
Petoskey stones can be found from the shores of Traverse City, north to the Charlevoix and Petoskey area, and across the state to Alpena, but the most popular place to hunt for them is at Lake Michigan beach. Spring is a good time to look for the stone after the ice has melted and uncovered specimens that they’ve pushed against the shore.
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