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Michigan designated Isle Royale greenstone (chlorastrolite) as the official state gem in 1973. Chlorastrolite (a variety of the mineral pumpellyite) is a bluish-green stone with a pattern of slender, star-like crystals which results in a "turtle back" pattern. Some chlorastrolite includes other minerals, such as on three of the stones pictured above (the pink parts are identified as thomsonite).
Photo of polished Isle Royale greenstone © John Jaszczak:
The Mineral Museum of Michigan - A. E. Seaman Mineral Museum (used by permission)
Isle Royale greenstone (sometimes called green starstone or turtle back in Michigan) is usually found as bean-sized, rounded beach pebbles (collecting chlorastrolite from Isle Royale National Park is now prohibited). Polished stones are used for stickpins, rings, earrings, cufflinks, pendants, and sometimes incorporated into inlays and mosaics.
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