Opals are extraordinary stones, their optical properties creating infinite variety - each has a pattern of bright iridescence all of its own, as unique as a fingerprint. This iridescence is the product of light interacting with a minute network of spherical structures of silica within the opal, and the resulting flash of colour varies with the density and size of these groups of spheres. The most commonly seen colours are blue and green, while the rarest are at the other end of the spectrum: orange and red. The patterns in which these colours appear are also varied - small flashes of colour are known as ‘pinfire’ opals, while a patchwork of broad flashes of contrasting colours are particularly prized, known as ‘harlequin’ opals. The body colour of opal can also vary in both shade and transparency, including white, black, and a vivid orange variety known as ‘fire opal’. The birthstone of october, opals occur in several varieties, notably across Mexico, Ethiopia and most famously Australia.