Beachcombers on the shores of the Great Lakes have a good chance of coming across a strange find: a rock with a strange, hexagonal pattern on its exterior. These aren’t just rocks, but Petoskey stones, a combination of fossil, rock, and coral. Though they aren’t particularly rare (they’re the state stone of Michigan, after all), they are still a fascinating find.
They are the favorite find of Cassandra Tiensivu, who beachcombs along the rocky shore of Lake Michigan near the city of Petoskey. She’s a fossil hunter who loves finding a Petoskey stone, which doesn’t technically become a Petoskey stone until it is cleaned and polished. Until that point, the stones are called Hexagonaria coral.
Zoanthus coral polyps in an aquarium, a modern cousin to Hexagonaria coral.
Petoskey stones can be found all across the state of Michigan, and are easiest to spot when wet (otherwise the distinct pattern stays hidden). The stones were once living organisms called rugose corals, and massive colonies of these corals lived under a tropical sea that covered Michigan 350 million years ago.
Learn about identifying your Great Lakes fossil finds.
Magnus Park in Petoskey has been particularly fruitful for Cassandra’s beachcombing searches. She likes going early in the morning and specifically after a storm, as fresh fossils will likely have washed ashore. If you plan on wading into the water, she recommends going on an overcast day to avoid the glare of the sun.
“This is pure speculation on my part, but I believe that there must be large chunks of these fossilized corals sitting at the bottom of Lake Michigan. Over time, smaller pieces break loose and the currents and waves push them toward the shore,” says Cassandra. “It is the only logical conclusion I can come to based on the fact that I visit the same spots year after year, and the supply always seems to be replenished.”
“Petoskey stones polish up beautifully with grinding wheels. However, as someone who does not have access to more professional lapidary equipment myself, I developed my own hand polishing method that yields amazing results,” Cassandra explains. “I use ten different grit wet/dry sandpapers (100, 220, 320, 400, 600, 800, 1000, 1500, 2000, and 2500), along with a Sunshine Polishing Cloth.
Keep in mind that polishing dry is dangerous for your lungs, so you should always submerge your stone in your bucket of water when polishing, or at least have it underneath some sort of running tap. I start with the roughest grit (100), and work all the way up to the finest (2500). Then, when I’m satisfied with the overall appearance, I will use my polishing cloth to really bring out that final shine.”
Once polished, Cassandra puts hers on display in her home or turns them into wire wrapped necklaces. Cassandra adds, “I’ve seen others make some beautiful knives with Petoskey stone handles, wine stoppers, kitchen backsplashes, framed rock art…the possibilities for what you can use them for are limited only by your own imagination.”
Cassandra started collecting stones about ten years ago, though she picked up many treasures during casual lake visits in her tween years. She used to beachcomb with her mother, and today, she beachcombs with her teenage daughters.
Cassandra has collected hundreds of Petoskey stones over the years, ranging from the size of her palm to giants weighing up to twelve pounds. Her favorite finds are Petoskey stones with strange patterns — she has one with a cat-like creature on the front, one with a creeping spiral, and a multicolored stone she calls a patchwork Petoskey. Cassandra says that she wants to complete an entire rainbow of colors with Petoskey stones (which most often show up in grays or browns). In her searches in Michigan, she also found a fossil that looked like it was covered in dragon scales, which turned out to be another type of coral fossil called horn coral. She would love to find one of the state gemstones, chlorastrolite, which is also known as greenstone.
“In Michigan, there is an annual limit of 25 pounds per person that can be legally taken from state-owned public beaches. It only takes me one or two day trips during the summer to reach that limit,” says Cassandra. “I also tend to refrain from going more often to allow others a chance to find their own special treasures. Plus, if I’ve already hit my limit, I’m afraid I might find something that I really want to take home, but can’t!”
“If I am out beachcombing when other families are out with their children, I often end up with little shadows interested in what I’m doing,” laughs Cassandra. She shows them her finds and helps them search for and identify their own. If they don’t happen to find any for themselves, she gives them a few of hers before they leave. “I will also occasionally gift a few to folks from out of state that have stopped by to look for Petoskey stones while I’m hunting,” she continues. “I hand polish the remaining stones to either add to my collection, make into necklaces, or give as gifts.”
All Petoskey stone photos courtesy of Cassandra Tiensivu. See more of Cassandra’s collection on Instagram @cass616 and view a tutorial on hand polishing Petoskey stones at youtu.be/dBw3ztN9CFc.
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Learn more about fossils found on modern and ancient shores around the world. Articles ›
Beach fossils can be found on beaches all around the world
This article appeared in the Beachcombing Magazine November/December 2019 issue.