Pumpellyite var. Chlorastrolite

specimen number:
2062045

location:
Cliff Mine, Keweenaw Co., Michigan, USA

description:
A polished example of what is known as chlorastrolite, which is pumpellyite.

Chlorastrolite is a rare variety of the mineral pumpellyite that occurs in cavities within basalt lava flows of the Keweenaw Peninsula in northern Michigan. It is best known as the official state gem of Michigan and is commonly called the Lake Superior stone. The mineral typically forms rounded nodules that, when cut and polished, display a distinctive radiating green pattern that resembles a turtle shell, caused by its fibrous internal structure. Most chlorastrolite is found along the shores and underwater areas of Isle Royale and the Keweenaw Peninsula, where erosion of the basalt has released the nodules.

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