The beryl group of silicates includes the important gem varieties emerald, blue aquamarine, pink morganite and red and yellow beryl which have no regular names. Other colours sometimes turn up and the commoner ones may be pale or strongly coloured. The formula usually given for the beryl group is Be3Al2Si6O18; however, the general formula may be expressed as A2-3B2Si5(Si,Al)O18, where A beryllium, magnesium or iron and B Aluminium scandium or iron. The mineral crystallizes in a hexagonal or in a closely related orthorhombic space group (Dana’s New Mineralogy, 1997). Cordierite (iolite), is also a member of the beryl group but in this section we are looking at the varieties of beryl itself. Crystals are usually columnar hexagonal prismatic with a basal pinacoid; some emeralds may show second-order prisms. Recently the new mineral pezzottaïte has been found in Madagascar
Emerald Stone is coloured by the trace element chromium though vanadium is often present; attractive green beryls which can closely resemble emerald may be coloured by traces of vanadium alone. Some alkalis may be present but do not usually affect beryl’s ornamental properties. Beryl is a ring silicate in which the rings are centred on hexagonal axes; the rings consist of six silicon–oxygen tetrahedra. The colouration and properties of the other ornamental beryls are discussed below.