Opal

Opal is hydrated amorphous silica, containing between three and twenty percent water. It has no crystal structure, which makes it a mineraloid rather than a mineral, and the water content makes it unstable: opal can craze, developing a network of fine cracks, if it dries. Ethiopian hydrophane opal absorbs water and changes appearance temporarily when wet, which is disconcerting and reversible.

The play of colour was not explained until the 1960s, when electron microscopy showed that precious opal consists of silica spheres packed in a regular three-dimensional array. Light diffracts from this array exactly as it does from a crystal lattice, and the colours seen depend on the sphere diameter and the viewing angle. Larger spheres give reds, smaller give blues; a stone showing red generally shows the whole range, which is why red-dominant opal is the most valuable. Common opal, with irregularly sized spheres, shows no play of colour at all.

This is one of the more satisfying explanations in gemmology: an optical phenomenon that looks like an internal fire turns out to be diffraction grating physics, and the structure responsible is invisible at any magnification available before the twentieth century.

Australia produces the great majority of the world's precious opal, principally from Lightning Ridge, Coober Pedy and Andamooka, and the Australian fields were only developed from the 1880s onward. Before that the source was Hungary, in what is now Slovakia, which supplied the opals known to the classical and medieval world. Ethiopian material entered the market in quantity from the 2000s.

Opal has one of the sharpest reputational reversals of any gem, and both ends of it are documented.

Pliny writes about opal with unusual enthusiasm, describing it as combining the fires of many stones (the flame of carbuncle, the purple of amethyst, the green of emerald), and tells the story of the senator Nonius, who was proscribed by Mark Antony and chose exile over surrendering a famous opal ring. Whatever the truth of the anecdote, the stone was highly valued in Rome.

Its reputation for bad luck is modern and traceable. The usual account attributes it to Walter Scott's novel Anne of Geierstein, published in 1829, in which the Lady Hermione wears an opal that flashes when she is agitated, is extinguished by holy water, and she is reduced to ashes. The book was widely read, and opal prices are reported to have fallen substantially in the following years.

That account is itself worth handling carefully. It is repeated in every gem book, it originates in George Kunz's The Curious Lore of Precious Stones of 1913, and the claim that the novel single-handedly crashed the market is an assertion by a well-informed writer who was also the chief gemmologist of Tiffany and had a professional interest in dismissing the superstition. The direction of the effect is plausible; the magnitude is not independently established. There is also a simpler contributing factor: opal is genuinely fragile and prone to crazing, and jewellers dealing with returned and damaged stones had their own reasons to spread a caution.

Opal is the October birthstone; pink tourmaline was added alongside it in 1952.