Chlorastrolite

Chlorastrolite, also known as Isle Royale Greenstone, is a green or bluish green stone.^[1] Chlorastrolite has finely radiating or stellate (for examples, see crystal habits) masses that have a "turtleback" pattern. The stellate masses tend to be chatoyant, meaning they have a varying luster. This chatoyancy can be subtranslucent to opaque. Cholorastrolite is a variety of pumpellyite: Ca
₂(Mg,Fe)Al
₂(SiO
₄)(Si
₂O
₇)•(OH)
₂H
₂O. Chlorastrolite was once thought to be an impure variety of prehnite or thomsonite.

^ Arguably the rarest gemstone in the world and one of the most valuable American gemstones. The gem quality specimens are typically found on Isle Royale, a large island off the Northeastern tip of Minnesota. a Michigan's State Gemstone

[1] Arguably the rarest gemstone in the world and one of the most valuable American gemstones. The gem quality specimens are typically found on Isle Royale, a large island off the Northeastern tip of Minnesota. a Michigan's State Gemstone

[1]