Silicate of beryllium and aluminium.
Crystal system Hexagonal.
Appearance Beryl crystallizes as fairly complete hexa gonal prisms, sometimes with basal faces or small blpyramidal facets. The crystals can be very large—from a few centimeters, up to some tens of centimeters—with occasional specimens over a meter in length. Beryl is often cloudy and, when transparent, has a vitreous luster. It ll usually an opaque, milky white, or a faint yellow, very pale gray or green color. Stronger, more attractive colors, however, also occur: mainly blue, green, yellow or pink. Al- though rare, red and colorless specimens do exist.
Crystals that combine brilliant color with transparency are highly-prized as gems.
Physical properties It has a hardness of 7.5 to 8, but In fairly brittle, and may show ill-defined cleavage parallel to the basal plane. The density is normally 2.67--2.72 g/cm”, but can be as much as 2.90 g/cma. The refractive indicea, like the density, are somewhat variable, from ﬁe 1.560, nw 1.570 to FIE 1.595, nw 1.602.
Genesis Beryl occurs as an accessory mineral in granite rocks, and crystallizes mainly in pegmatites,‘ where the largest individual crystals are found. It is also formed by metasomatism in the country rock surrounding pegmatitee, and is associated with hydrothermal processes.
Occurrence Beryl is very widespread; it occurs in pegmatites in many areas from northern Europe to North America, South America, East Africa, South Africa, and Himalayan Asia. It occurs also in calcite veins, as a process of hydrothermal activity in the bituminous limestones of Bogota, Colombia.