Michigan semi-precious Greenstone is found only in Michigan’s Keweenaw Peninsula and on Isle Royale National Park. The first documentation of this stone was in 1847 by Drs. C. Jackson and J. Whitney. The official name, chlorastrolite, was designated as Michigan’s official state gem in 1973.
Chlorastrolite is a bluish-green stone that has a segmented pattern resembling a turtle’s shell. Turtle Back and Green Star Stone are some other names that have been applied to this uncommon stone that is not found anywhere else in the world. Sometimes chlorastrolite is found with with other minerals inbedded in it.
Chlorastrolite was formed in gas bubbles that formed deep within the basalt layer under the Keweenaw Peninsula. Isle Royale was formed off-shore from Keweenaw by an upsurge of bedrock under Lake Superior. This upsurge of bedrock exposed Greenstone in the basalt. The Greenstone broke loose with wave action and soon showed up in the beach gravel along the shores of the Isle.
Chlorastrolite (Greenstone) usual size varies between 1/8″ and 1/2″. Quality is usually poor in larger stones. It is difficult for lapidaries to work with. Some chlorastrolite stones are hollow or calcite-filled, making them break easily. But with patience and carefully applied polishing pressure, lovely treasures can be accomplished. The Smithsonian has the largest chlorastrolite in existence, measuring 1-1/2″ x 3″.
If you find yourself in Michigan’s Upper Peninsula, make sure to visit some rock shops to pick out one of these beautiful semi-precious stones. They make beautiful pendants, inlays, rings, etc. The scarcity of this stone is growing rapidly. And please remember, it is now illegal to harvest Greenstone in Michigan’s Keweenaw Peninsula and on Isle Royale National Park.