Chlorastrolite, also known as Isle Royale Greenstone, is a green or bluish green stone. 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.