I was standing outside a shop in Traverse City gazing at rocks in the window display when I overheard two people.
“Let’s go in here,” one said. “I want to see those Petoskey rocks.”
I chuckled to myself. I didn’t have to look to know they weren’t from here. Everybody admires the beautiful patterns revealed in a polished Petoskey stone.
We’re used to seeing them in a horizontal cross section, but I got to wondering what did those little creatures look like in a vertical cross section? Surely, they were not two-dimensional.
Turns out they were little tube-shaped corals, called hexagonaria that lived in colonies. The six-sided shape results from crowding. It’s the simplest geometric form that allows all the available space to be filled. Like honeycombs.
Each individual animal is called a polyp.
They were essentially skin-covered stomachs, with cells at the bottom that secreted calcium carbonate. Each polyp lived on top of its own skeleton. Periodically, the polyp detached itself from its base and created a new base on top of the old one. So the individual grew in height along with the reef the colony was building
Do you want to find some Petoskey stones? I’ve had the most success in landscaping rocks around trees, shrubs and other plantings. People don’t want to haul rocks any further than they have to, so rocks deposited in northern Michigan likely originated right here — home to the Petoskey stone.
From time to time, when I’m admiring a Petoskey stone, it occurs to me that these simple, little animals, that lived more than 300 million years ago, have made a more lasting impression than I ever will.