Beryl crystal structure and symmetry, illustration. Beryl is a cyclosilicate mineral built around 12-membered crown-shaped rings of six tetrahedral silicon atoms (blue grey) alternating with six bridging oxygen atoms (lightest blue). Each silicon atom is also bonded to two other oxygen atoms that are shared with beryllium(2+) and aluminium(3+) cations. These metal ions form a second ring that encloses the first. The outer ring has six aluminium oxide (AlO6) octahedra (dark blue) alternating with six tetrahedral beryllium oxide (BeO4) groups. The rings create channels that contain Group 1 cations, caesium(+) (larger) and sodium(+) (darker). Pure beryl is colourless, but trace amounts of impurities give coloured gemstones. Aquamarine, is a pale blue form of beryl contaminated with iron(2+) in some of the aluminium(3+) sites. The green emerald has chromium(3+) impurities; iron(3+) impurities give heliodor, a golden form of beryl, and morganite is pink due to manganese(2+).