Beryl crystal structure and symmetry, illustration. Beryl is a cyclosilicate mineral built around 12-membered crown-shaped rings of six tetrahedral silicon atoms alternating with six bridging oxygen atoms (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 (green) alternating with six tetrahedral beryllium oxide (BeO4) groups (olive green). The rings create channels that contain Cs(+) (larger) and Na(+) (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+).