Opal

Turn an opal in your hand and flashes of red, green, blue and violet move across it. That play of colour comes from tiny spheres of silica stacked in a regular grid. Light bends between them, and the size of the spheres decides the colour you see.

For more than a century nearly all precious opal came from Australia, which still produces most of the world's supply. Since 2008 Africa has had its own great source: the highlands of Wollo in northern Ethiopia.

Where the name comes from

The name probably comes from the Sanskrit upala, "precious stone", which became opalus in Rome.

Beliefs and legends

• Fallen from the sky. Arabic legends say opal falls from the heavens in flashes of lightning.
• Hope and prophecy. The ancient Greeks believed opal gave its owner the gift of prophecy; to the Romans it stood for love and hope.
• The luckiest gem. For most of history opal was thought the luckiest and most magical of gems, because it shows every colour. Its reputation for bad luck comes from a novel: Sir Walter Scott's Anne of Geierstein, published in 1829.

Two kinds of opal: Ethiopian and Australian

Ethiopian opal is found mainly in volcanic rock, high on the plateaus of Wollo, about 550 kilometres north of Addis Ababa. It is mostly white or clear with bright play of colour, and there is black opal too. Most Ethiopian opal is hydrophane: it can absorb water, turning clearer for a while and losing some of its colour until it dries.

Australian opal formed in sedimentary rock, in the dry inland basins of the outback. Lightning Ridge produces most of the world's black opal; Coober Pedy, Andamooka and White Cliffs are known for light opal; and boulder opal, a thin layer of opal on ironstone, is found only in Queensland.

The types, and their names

• White or light opal: play of colour on a pale body.
• Black opal: play of colour on a dark body, which makes the colours glow.
• Crystal opal: clear, with strong play of colour.
• Boulder opal: opal left on its ironstone, found only in Queensland.
• Fire opal: transparent yellow, orange or red, often without play of colour.

Treatments, honestly

Some opal is impregnated with oil, wax or plastic, and some is darkened by "sugar" or "smoke" treatments to imitate black opal. Hydrophane Ethiopian opal takes dye easily, and dyed stones have been sold under other names. A doublet or triplet is a thin slice of opal glued to a backing, sometimes with a clear cap. These are honest, affordable pieces when they are sold as what they are. We tell you whether an opal is solid, a doublet or a triplet, and what we know of any treatment, before you buy it.

How to judge an opal

• Play of colour first: its brightness and its pattern.
• Body tone: a dark body makes the colours stand out.
• Solid or assembled: ask whether it is solid, a doublet or a triplet.
• Shape follows the stone: fine opals are often cut to irregular shapes to keep as much colour as possible.

Caring for your opal

Opal is soft (5 to 6.5 on the Mohs scale) and fragile. It suits pendants and earrings best; in a ring, wear it with care. Store it apart from harder gems, which will scratch it. Clean it only in warm soapy water. Keep it away from high heat, sudden changes of temperature and intense light, which can cause fine cracks called crazing. Do not soak a doublet or triplet: water can weaken the glue.

At a glance

Mineral	Opal (hydrated silica)
Hardness	5 – 6.5 Mohs; fragile
Types	White, black, crystal, boulder, fire; Ethiopian opal mostly hydrophane
From	Wollo and Shewa, Ethiopia; Australia (Lightning Ridge, Coober Pedy, Queensland)
Treatments	Some oiled, dyed, sugar- or smoke-treated; doublets and triplets. Disclosed per stone
Care	Warm soapy water only. No heat, no sudden temperature change