It’s a fossil coral. Why is it called a Petoskey Stone? Because many of their kind are found abundantly in and around the Lake Michigan shores near the northern city of Petoskey, Michigan (USA)!
They are also called “lucky stones” so it’s really great to find one!
How could the remains of a coral which thrived in tropical warm waters possibly find its way to Michigan? Because 350 million years ago during the Devonian Time Period much of North America was covered by warm shallow seas. Later, the corals were buried under deep layers of sediment. Millions of years after that, when the great glaciers retreated, they scraped and dug into those forgotten layers of earth. The glaciers deposited them where we can now enjoy the good fortune of discovering their mysteries.
Petoskey Stones were mass coral colonies of Hexagonaria, percarinata. Each hexagon (very visible in the stone) held a single animal which opened a mouth exposing its tentacles in order to take in food. The tentacles were also used to sting any organism or other corallite that came too close. Calcite, silica and other minerals replaced the original corallite exoskeleton over many millions of years.
Common Name: Petoskey Stone or Lucky Stone
Scientific Name: Hexagonaria, percarinata
Phylum: Cnidardia (means to sting)
Class: Anthozoa (ie coral, sea pens, sea anemones)
Subclass: Zoantharia (true corals)
Order: Rugosa (means wrinkled wall)
Family: Hexagonaria (means six sides)