Greenstone (9.0 cm across at its widest)
Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy.
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).