Opal (Australian, Boulder, Black & Fire)

Unlike most gemstones, opal is amorphous - it lacks a rigid crystal lattice and consists of hydrated silica spheres. Precious opal exhibits play-of-colour, an optical phenomenon in which light diffracts through a tightly packed grid of microscopic silica spheres to produce flashes of spectral colour. Black opal (primarily from Lightning Ridge, Australia) is the most valuable variety: its dark body tone provides a stark contrast that makes the colour flashes stand out. Fire opal, mined primarily in Mexico, is a distinct type; it is prized for its transparent-to-translucent orange-red body colour and often lacks play-of-colour entirely, which allows it to be faceted rather than cut as a cabochon.

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