Greenstone is one of several greenish-colored metamorphic rocks. It is typically composed of epidote, chlorite, and plagioclase feldspar. Greenstones are supposed to have a crystalline texture, but some are weakly foliated (but not enough to be called schist). Greenstones are the result of low-grade to intermediate-grade metamorphism of basalts or gabbros. They are relatively common rocks in greenstone belts in Precambrian shield areas. Many greenstones still preserve physical aspects of the original basalt lavas (for example, vesiculation or pillow lava geometry). Some greenstones have been interpreted to be metamorphosed graywackes (dirty sandstones rich in mafic minerals).
Greenstone (9.0 cm across)