Petoskey Stones of Michigan



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.


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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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Sources: Wikipedia / / Michigan Science /

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  1. When I was a kid vacationing in Michigan a local - a high school science teacher - convinced us that these stones were made of dinosaur poop. I like the original story better!!

    1. It's not about "stories"'s about facts! It's really frightening to think that this guy was a high school science teacher!

  2. The really remarkable thing about Petoskey stones is that I had never heard of them before I found 4 of them lying on the eastern shore of Carlyle Lake, in Southern Illinois some years ago! I owned a small farm in the area and was a short walk from the shore which had no pubic access on most of the shoreline. I took the dog down to play in the water one morning and I saw an odd patterned rock lying in the shallow water. I picked it up and examined it and decided to keep it for a conversation piece. When I got home and pulled the then dried rock from my pocket, it looked like just another plain rock. I did a search on the internet for "honeycomb patterned rock" and one of the hits was a link to an explanation of the Petoskey stone.

    I just figured that someone had dropped it in the sand accidentally while walking on the beach until some weeks later I found another stone of the same internal construction! In all, I found 4 of these rocks in the following year or so and there may have been many more that I hadn't yet come upon.

    The mystery of how they got from the Upper MI area all the way down to Southern IL was solved when I spoke to a friend of mine who taught geology at the nearby community college. He told me that when the glaciers pushed South many thousands of years ago, they had stopped their Southern movement and began receding back North again right about where we lived in So. IL. As they melted, they left deposits of rocks and debris that was pushed ahead of the glacier mass, called "tailings", and he surmised that the stones that I found on the shore of Carlyle Lake just off the North edge of my farm must have been pushed all the way from Northern MI down to So IL and the natural erosion of the lake had worn the clay and sand from their resting place and brought them to the surface for me to find!

    It sure makes for an interesting story on a cold winter's night! I feel so lucky to have stumbled onto such treasures so far from their original home. They now have a place amongst the other fossils, pottery shards, arrowheads, and spear points that I have found in the general area.

    The old adage is, "You can't see the forest for the trees" and my adaptation of that saying is, "You CAN see the arrowheads if you look down once in awhile". The view is great but one misses the beauty that is right in front of them if they don't look down occasionally.

  3. Great story Framer. My son and I have found Petoskey stones far inland in southern Michigan. They are awesome. in fact, we just got back from Petoskey MI. 10/20-23/16 where we found over 150 nice stones from the beach, and inland along the Bear River... which drains into Little Traverse Bay. Needless to say, no matter where we go in Michigan... if we find a rock pile we look for Petoskey stones. Blessings!


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