Petoskey stones

Dry, it looks like a boring grey pebble. Wet, it turns into a 350-million-year-old coral reef.

A Petoskey stone is a collectible rock made of fossilised rugose coral, the species Hexagonaria percarinata. They are usually pebble-shaped, and glaciation is why: sheets of ice plucked the fossil corals out of the bedrock, ground the rough edges off them and dropped them across the north-western part of Michigan's lower peninsula, with some in the north-east. In those same areas you can still find complete fossilised coral colony heads in the source rocks.

The rock they come from is the Gravel Point Formation of the Traverse Group, and the reef itself was laid down in the Devonian, roughly 350 million years ago. That is the whole appeal: when the stone is dry it passes for ordinary limestone, and when it is wet or has been polished with lapidary tools the mottled pattern of six-sided coral cells appears. People cut them into decorative objects and set them as gemstones. Other fossil corals turn up in the same places, so not every patterned pebble is a Petoskey.

Michigan named it the state stone in 1965.

There is a whole day built around it. The Petoskey Stone Festival, now in its twentieth year, runs at Barnes Park Campground in Eastport with live music, food, an arts and craft show full of handmade Petoskey stone work, and a button design contest. The 2026 event is set for 16 May.

The practical trick is water. Carry a spray bottle or work the wet edge of the shoreline, because a dry Petoskey stone is genuinely hard to pick out from the shingle around it. Check the rules wherever you are walking: collecting limits apply on Michigan state land, and taking stones from national lakeshore ground is a different question again.