Opal

Turn an opal in the light and colours flash across it: green, blue, orange, sometimes red. That play of colour comes from tiny spheres of silica, stacked in neat rows inside the stone. Light bends between them and breaks into a rainbow, and the size of the spheres decides which colours you see.

Opal is not a South African gem. Africa's opal was found in 2008 in the Wollo highlands of Ethiopia, in layers of volcanic rock high above the valleys. Ethiopia is now one of the world's important sources of opal, though Australia's fields still produce the most.

The kinds of opal

• White opal: play of colour on a light body.
• Black opal: play of colour on a dark body, which makes the colours glow. Ethiopia has produced black opal since 2013.
• Crystal opal: clear to translucent, with colour seen deep inside.
• Fire opal: orange to red, with or without play of colour.

Ethiopian opal and water

Most Ethiopian opal is hydrophane: it soaks up water and can turn transparent when wet, then return to normal as it dries. So keep it away from water, oils and lotions.

Beliefs and legends

The ancient Greeks linked opal with prophecy and the Romans with love and hope; for centuries it was thought the luckiest of gems. Arabic legends said opals fell from the heavens in flashes of lightning. Its later name for bad luck came from a novel, Sir Walter Scott's Anne of Geierstein (1829).

Treatments and doublets, honestly

Some opals are impregnated with oil, wax or plastic, or darkened with sugar or smoke treatments. A doublet or triplet is a thin slice of opal glued to a backing, sometimes under a clear quartz cap, and must always be named as such. Any treatment we know of, and every doublet and triplet, we tell you.

How to judge an opal

• Play of colour is everything: its brightness, the range of colours, and how it covers the face.
• Body colour: a dark body makes the colours look stronger.
• Shape: opal is often cut in free shapes to keep as much colour as possible.

Caring for your opal

Opal is 5 to 6.5 on the Mohs scale, soft and fragile. Wear it in pendants and earrings, or in a ring with a protective setting. Clean it only in warm soapy water. Keep it away from high heat and sudden changes of temperature, which can crack it, and store it alone so harder gems cannot scratch it. Long soaking can weaken the glue in doublets and triplets.

At a glance