Chlorastrolite, also known asIsle Royale Greenstone, is a green or bluish green stone.[1] Chlorastrolite has finely radiating or stellate (for examples, seecrystal habits) masses that have a "turtleback" pattern. The stellate masses tend to bechatoyant, meaning they have a varying luster. This chatoyancy can be subtranslucent to opaque. Cholorastrolite is a variety ofpumpellyite:Ca
2(Mg,Fe)Al
2(SiO
4)(Si
2O
7)•(OH)
2H
2O. Chlorastrolite was once thought to be an impure variety ofprehnite orthomsonite.
Chlorastrolite occurs asamygdaloid structures and fracture fillings inbasalt, and when the water and wave action has worn away the basalt, they are found as beach pebbles and granules in loose sediments. It is found in theKeweenaw Peninsula of theUpper Peninsula of Michigan andIsle Royale inLake Superior. Isle Royale is a National Park, and so it is illegal to collect specimens there. It is difficult to identify an unpolished pebble of chlorastrolite. Most gem quality chlorastrolite stones are very small, and it is rare to find one that is larger than a half inch. The largest gem quality stone is in the Smithsonian Museum and measures 1.5 by 3 inches.
It was first described from Isle Royale, Lake Superior by C. T. Jackson and J. D. Whitney in 1847. Chlorastrolite, also known as greenstone, is the officialstate gem ofMichigan.[2]
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