A professionally polished Petoskey stone, similar to what you would find for sale in shops throughout Michigan. The diagram in the corner indicates scale. Photo by Dr. Mark Wilson of the College of Wooster, Ohio, via Wikimedia Commons.
This summer, transport yourself to the eastern shore of Lake Michigan, near the town of Petoskey, and perform a geological magic trick. Take a chunk of limestone on the beach – bland, grey and completely featureless – and dip it into the lake. Stare into the translucent depths at your chosen stone and watch distinctive hexagonal patterns emerge on the surface. But a funny thing happens when you pull the stone out of the water and let it dry in the sun – the markings abruptly vanish! What’s going on here?
As you might have suspected by now, this is no ordinary piece of limestone. Instead, you’ve stumbled across a Petoskey stone, beloved by geologists and beachcombers alike. Like all magic tricks, however, there is indeed a simple explanation for the vanishing act, impressive as it appears on first viewing. In this case, our mysterious rock’s origins in the distant past owe just as much to the animal as the mineral.
Four hundred million years ago in the Devonian era, Lake Michigan did not exist, but there was no shortage of water in the area. A wide, shallow tropical sea covered the area, home to a diverse array of bizarre-looking marine life: horseshoe crab-like trilobites, spiraling ammonites, brachipods and other shellfish, the first jawed fishes, sprawling crinoids and crustaceans. (When T.S. Eliot has J. Alfred Prufrock mutter, “I should have been a pair of ragged claws / Scuttling across the floors of silent seas,” he was probably thinking of the Devonian.) It was also a golden age for coral, including one family – now extinct – known as rugosans, or “wrinkled,” on account of their distinctive appearance.
While many of these rugosan corals were solitary polyps, other species opted for a more communal approach. They massed together, forming vast reefs similar to the ones we see in our own era off Hawaii or Australia. One of these colonial coral species, Hexagonaria pericarinata, was particularly abundant around the future home of Petoskey, Michigan. As the scientific name suggests, these corals were highly symmetrical, with each animal forming distinctive six-sided walls as it tried to occupy the maximum possible space against its many neighbors. (Unlike the rigid chambers you see in honeycombs, you’ll often find five, four, or even three-sided corals who just couldn’t grow up fast enough to compete with their more aggressive counterparts, or asymmetrical hexagons formed by polyps that got “squashed”.)
When individual polyps died, others grew on top of them and the cycle repeated. Eventually, whole reef conglomerates were buried by ocean sediments in the shifting seas. Time and pressure transformed the coral and sediment matrix into calcium-rich limestone, the coral polyps preserved as flattened outlines of their former, three-dimensional selves in the rock. Each polyp’s alimentary canal became a dark speck that forms the “eye” of each hexagon; the lines that radiate outward from the center mark where tentacles were located.
Fast forward to twelve thousand years ago in the Pleistocene. The tropical ocean vanished long ago, replaced by bare stone and a wall of glaciers three miles high. As the glaciers advanced and retreated, they scoured the bedrock, kicking up chunks of the limestone reefs and gouging out the depression that would fill with meltwater and become Lake Michigan. Rounded and sculpted by water, the fossilized coral fragments were moved by centuries of robust freeze-thaw cycles southward in the lake, occasionally washing up on beaches tens or even hundreds of miles away from their original site.
Dubbed “Petoskey stones” because of their abundance on the beaches and bedrock of Petoskey, Michigan, these stone corals are ubiquitous in gift shops throughout the state. You can find their likeness on a number of “Petoskey tchochkes” including magnets, bumper stickers and t-shirts, so residents and tourists alike can proclaim their love. As a sign of their distinctive popularity, Petoskey stones have been the official state stone of Michigan since 1965. They remain a source of local pride and a prize find during any lakeside stroll. Most stones you find on the beach are tiny, but huge storms can wash five, ten, or even twenty pound specimens out of the lake overnight.
Why are they easier to spot in water? The contrast between the slightly lighter crystals of fossilized coral body (calcite) and the surrounding stone is greatest when wet, making them easiest to detect underwater or well within the splash zone of the lake. (Further away from the lake shore, less discriminating rockhounds opt for spittle or other body fluids as a way to test their finds.) Some stones will naturally keep their appearance without water or any further treatment, but most of the stones available for sale have been professionally polished to ensure that the distinctive hexagons remain visible even when dry.
You can replicate the same look on stones you find on the shore with some fine-grit sandpaper, elbow grease, and a little toothpaste and velvet cloth for polish. Limestone is soft and easily eroded, so a little bit of effort goes a long way – a conventional rock tumbler is overkill, converting beautiful stones into a dusty pile of sediment. But maybe you’ll keep any stones you encounter in their natural state, tucked away in your pocket, ready to perform a magic trick of your own.