Wikipedia
Chlorastrolite
Chlorastrolite also known as Isle Royale Greenstone, is a green or bluish green stone. Chlorastrolite has finely radiated or stellate masses that have a "turtleback" pattern. The stellate masses tend to be chatoyant, meaning they have a changeable luster. This chatoyancy can be subtranslucent to opaque. Cholorastrolite is a variety of pumpellyite: Ca(Mg,Fe)Al(SiO)(SiO)(OH)·HO. Chlorastrolite was once thought to be an impure variety of prehnite or thomsonite.