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Petoskey Stones

Bill Vossler photo

Bill Vossler photo

Story by Bill Vossler

At 350 million years of age, Petoskey stones are older than the dinosaurs, which roamed the earth beginning 165 million years ago. Like dinosaurs, Petoskey stones are rocks and fossils (The Complete Guide to Petoskey Stones, Bruce Mueller and William H. Wilde, Petoskey Co-Pub, 2004). Though evidence of dinosaurs can be found almost everywhere around the world, it is not so with Petoskey stones. Mostly, they are found in Michigan, though some show up in Illinois, Indiana and Iowa.

Petoskey stones began life as coral planulae, free-swimming larva that propel themselves around in the warm waters of tropical seas with tiny hairlike projections. Eventually, they drop to the bottom of the sea. There, they anchor onto something solid and progress into the polyp stage, growing and dividing, making exact copies. To protect themselves, they secrete calcite to make a hard, cuplike outer covering, or exoskeleton, say Mueller and Wilde. Like a snail, the soft-bodied coral lives inside the hard exoskeleton and uses its tentacles to snatch food that drifts by.

[S]keletons of these tightly packed, six-sided corallites became fossilized, forming Petoskey stones. Their distinctive dark centers, called “eyes”, were actually the mouths of the coral. The distinctive lines surrounding the eyes that make the stones so attractive were once tentacles that brought food into the mouth (

For the full article, see the March 2017 issue.



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