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).

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.

Source:

Michigan Geological Survey: Dept. of Environmental Quality
Chlorastrolite: Central Michigan University
A. E. Seaman Mineral Museum: Mineral Museum of MI

Links:

Geological Symbols - 50 States List