Opal

What it actually is

Hydrated amorphous silica, SiO2 nH2O, with water content typically between three and twenty-one per cent. Hardness 5.5 to 6.5, no cleavage, and conchoidal fracture.

Play of colour comes from structure. Opal consists of silica spheres a few hundred nanometres across, and where those spheres are uniform in size and packed in a regular three-dimensional array they act as a diffraction grating. Sphere size determines which colours appear: smaller spheres give blues and violets, larger ones give reds. Common opal, without that regular packing, shows no play of colour at all.

That structure was only imaged and explained in 1964, at the CSIRO in Australia.

The water content is the source of opal's main vulnerability: crazing, a network of fine cracks, can develop with drying, heat or sudden temperature change.

Hydrophane opal, mainly from Ethiopia, absorbs water and can temporarily change appearance when wet, which is normal for that material.

Where it comes from

Australia has dominated world supply since the 1870s, producing white opal from Coober Pedy, black opal from Lightning Ridge and boulder opal from Queensland. Black opal from Lightning Ridge is the most valuable.

Ethiopia's Welo deposits, from 2008, produce large volumes of hydrophane material and have changed the market considerably.

Mexico produces fire opal, an orange to red translucent material that may or may not show play of colour.

Brazil, Peru, the United States and Slovakia, whose Dubnik mines were the historic European source, also produce it.

Opal forms from silica-rich groundwater in arid regions, filling voids in sedimentary rock, often replacing fossils. Opalised fossils from Lightning Ridge, including complete dinosaur and plesiosaur remains, are among the strangest objects in palaeontology.

What the traditions claimed

Opal was prized in Rome. Pliny describes it in glowing terms and records the story of the senator Nonius, who reportedly chose exile over surrendering an opal to Mark Antony.

Then in 1829 Walter Scott published Anne of Geierstein, in which the character Lady Hermione wears an opal that loses its fire when touched with holy water, after which she dies. Within a year or two the European opal market had reportedly collapsed, and opal carried a reputation for bad luck through the nineteenth century.

Queen Victoria pushed back deliberately, wearing opals and giving them as wedding presents, and the Australian discoveries from the 1870s supplied a market she had helped rebuild.

That sequence is the best-documented case anywhere of a stone's meaning being made and unmade by publication and patronage, and it is worth knowing before accepting any claim about a stone's ancient significance.

Modern practice reads opal as a stone of emotion, dream and amplified feeling, which is twentieth-century.

How it is worked with

The honest reading follows the physics: opal shows several colours at once and settles on none, which suits situations that are genuinely unresolved rather than merely undecided.

For wear, understand what you have. Solid opal can be worn with care. Doublets are a thin opal slice on a dark backing, and triplets add a clear cap; both are legitimate, much cheaper, and must never be soaked, because water destroys the adhesive.

Keep any opal away from radiators, car dashboards, ultrasonic cleaners and sudden temperature changes. Crazing, once it begins, cannot be reversed.

Hydrophane opal from Ethiopia will absorb water and go temporarily cloudy or transparent, then recover as it dries. That is normal, though it means such stones should be kept away from oils and dyes, which they also absorb.

Buying it honestly

Synthetic opal has been produced since the 1970s and shows a distinctive columnar or lizard-skin structure under magnification, along with an unnaturally regular colour pattern.

Doublets and triplets are legitimate assembled stones that must be disclosed. Viewed from the side, the flat junction and the dark backing are visible; in a closed setting they can be hard to spot.

Opalite is manufactured glass with a milky sheen and no play of colour at all, sold under names including sea opal and moonstone.

Plastic imitations exist.

For solid opal, the questions are origin and treatment. Some material is smoke-treated or sugar-treated to darken the background and enhance apparent play of colour, and that must be disclosed. Ethiopian hydrophane is particularly susceptible to dyeing, which is a growing problem.