Emerald is a beryl, which is a mineral composed of beryllium aluminium cyclosilicate. Other beryls include aquamarine, heliodor, phenacite, and morganite. It has a hexagonal crystalline lattice (something that shows well in this specimen). Emerald gets its deep green color from impurities of chromium and sometimes vanadium. It is considered a precious gemstone and is known for its inclusions and imperfections; flawless emeralds are pretty hard to come by.