The Isle Royale Greenstone

Michigan’s state gem is a rare, beautiful green stone with a “turtleback” pattern — and you’re not allowed to pick one up at the place it’s named for.

Chlorastrolite — Greek for “green star stone,” and known in Michigan as the Isle Royale greenstone — became the state gem in 1972. Its surface looks like a tiny turtle shell or a field of alligator scales, a mosaic of slender, radiating crystals that catch the light with a shifting, cat’s-eye shimmer. It was born of fire and deep time: it forms in the gas bubbles of ancient lava, a leftover of the Midcontinent Rift that tore at North America about 1.1 billion years ago — the very same volcanic event that filled the Keweenaw with copper.

Here’s the catch. Greenstone is found almost nowhere on Earth except Isle Royale and the Keweenaw Peninsula — and Isle Royale is a national park, where collecting anything is illegal. So the state gem can’t be gathered from its namesake island. Rock hounds hunt instead along Keweenaw beaches and the old copper-mine rock piles, where the little gems (often no bigger than a pea) turn up.