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This is a kind of triangular-shaped greenstone, about 11 mm x 11mm x 13mm. A large "star" covers about 1/4 of the stone, with nice chatoyant look throughout, giving the gemmy shine this treasure is known for.
The greenstone is also known as chlorastrolite which mean "green star stone". It was selected as the state gem of Michigan and is found only is small select areas of the Keweenaw Peninsula or on Isle Royale National Park.