Beryl. Photo source: Mineral Information Institute, courtesy of the Smithsonian Institution.
Emerald crystal from Muzo, Colombia.
Beryl is a mineral form of beryllium aluminum silicate (Be3Al2(SiO3)6), the chief ore of beryllium. It is usually yellowish, greenish-yellow, blue, or colorless. Only emerald, which is a variety of beryl, has the rich green color that makes it so valuable.
The finer varieties of beryl, which are transparent and of beautiful color, are distinguished as precious beryl, and are sometimes called aquamarine. These occur in crystals similar in form to those of emerald; but the regular hexagonal prism is more frequently modified by truncation on the angles or edges, acumination (sharpening to a point), etc. The prisms are often long with sides that are striated, often deeply so; but the truncating or terminating planes are smooth. The coarser varieties of beryl (common beryl) are also found crystallized, but often massive.
Beryl occurs chiefly as hexagonal crystals in veins that traverse granite or gneiss, or are embedded in granite; sometimes it is found in alluvial soils formed from those rocks. Hardness 8; relative density 2.6–2.8; melting point 1,400°C.
An emerald is a valuable green gemstone, a variety of beryl. Emerald's green color is caused by chromium, and occasionally by vanadium, impurities. Some of the best emeralds are mined in Colombia, Brazil, and Zambia.