Opal Jewellery Guide: Types, Quality and Care

Yes. Opal is a gemstone made of hydrated silica, silicon dioxide with water trapped in its structure, up to about 20%. Precious opal shows play of colour, flashes of spectral colour that move as the stone turns. Common opal has no play of colour and is used for beads and carvings. Opal is also the October birthstone.

Opal forms when water carrying dissolved silica seeps into cracks and hollows in rock and slowly evaporates, leaving the silica behind. In precious opal the silica settles as tiny spheres, about 0.2 microns across, stacked in regular rows. Light diffracts between them, which creates the flashes of colour.

Opal is about 5 to 6.5 on the Mohs scale, softer than quartz, so it scratches and chips more easily than most gems. Solid opal can be washed briefly in warm soapy water. Do not soak doublets or triplets, because water can loosen the glue, and keep hydrophane opal dry. Avoid sudden heat and cold.

Black opal has a dark body with bright colour; white opal a pale body; crystal or water opal is transparent with colour; boulder opal is a thin seam of opal on brown ironstone; fire opal is yellow, orange or red, with or without play of colour. Common opal, such as Peruvian blue and pink opal, has none.

Black opal with bright play of colour across the whole stone is the most prized, especially when it shows red, the rarest colour, and a bold pattern such as harlequin. Brightness, body tone, pattern, size and freedom from cracks all matter, and solid opals are valued well above doublets and triplets.

Australia has been the main source of precious opal since commercial mining began in the 1890s: black opal from Lightning Ridge, white opal from Coober Pedy and boulder opal from Queensland. Ethiopia has produced precious opal since finds in 1994 and 2008. Mexico is known for fire opal, and Peru for blue and pink common opal.

Ethiopian opal, mined mainly in the Wollo region since 2008, is often bright crystal or white opal with strong play of colour. Much of it is hydrophane: it absorbs water, turning temporarily clear or losing its colour while wet, and returns as it dries. Keep it away from water, oils and lotions.

A doublet is a thin slice of precious opal glued to a dark backing, which deepens its colour. A triplet adds a clear quartz or glass dome on top to protect the thin opal. Both are assembled stones, valued well below solid opal. Look at the side of the stone for straight join lines.

Yes. Opalite is a man-made glass or resin that shows a milky blue and amber glow, sold as a simulant of opal. It is not natural opal and shows no true play of colour. Synthetic opal, made by Pierre Gilson's process since the 1970s, is a separate laboratory material with opal's structure.

Opal is the October birthstone, shared with tourmaline, and is often associated with Libra. Its name comes from the Latin opalus. Opal has long stood for hope and creativity, and Victorian jewellers set it widely in brooches, rings and pendants, often with diamonds or black enamel.

Opal is soft and can crack from a knock or a sudden change of temperature, so an opal ring is best worn with care: take it off for gardening, sport and washing up. A protective rubover bezel and a raised diamond surround help. Opal pendants, earrings and brooches are safer for daily wear.

Natural opal shows play of colour that changes with the angle, in patches with irregular edges. Regular, honeycomb or snakeskin patterns point to synthetic opal, and an even glow without flashes points to glass. Check the side for the join of a doublet or triplet. A gem laboratory settles any doubt.

Opal is usually called a mineraloid rather than a true mineral, because it has no regular crystal structure: it is amorphous hydrated silica, made of tiny silica spheres with water between them. That structure is what gives precious opal its play of colour. Mohs hardness is about 5 to 6.5.

Yes. Precious opal can show blue flashes on any body colour, and some Ethiopian opal has a pink tone. Peruvian opal comes in soft blue and pink as common opal, with an even colour and no play of colour. Bright, uniform blue or pink stones sold cheaply may be dyed or glass.

Opal has been called both the luckiest and the unluckiest of gems at different times. There is no basis for the bad luck belief, and opal is the October birthstone, worn widely in Victorian and modern jewellery. The practical caution is care: opal is soft and should be protected from knocks.

Precious opal shows play-of-colour, flashes that shift as the stone or the light moves, so its colours change with the viewing angle. Some Ethiopian hydrophane opal turns clear or loses its colour when it absorbs water and recovers as it dries. An opal that loses water too fast can craze, cracking into fine lines that do not reverse.

Yes. Opal is hydrated silica, made of tiny silica spheres with water held between them, usually 6 to 10 per cent by weight. Precious opal shows colour because the spheres are regular in size and arrangement, which diffracts light. Hydrophane opals also absorb water from outside, which can change their look for a while.

No. Solid opal does not need to be kept in water or oiled, and oil can stain it or sit in fine cracks. Store opal away from heat and strong sunlight, apart from harder stones. Doublets and triplets must never be soaked, because water loosens the glued layers. Wipe opals clean with a damp soft cloth.

Common opal is plentiful, but precious opal with play-of-colour is rare, and black opal from Lightning Ridge is the rarest. Australia is the largest producer of precious opal and made it the national gemstone in 1993, with fields at Coober Pedy, Andamooka, Lightning Ridge and in Queensland. Ethiopia has been a major source since 2008.

Mexican fire opal is a transparent to translucent yellow, orange or red opal, mined mainly in Querétaro; some shows play-of-colour and some does not. Jelly opal is clear or nearly clear, with play-of-colour that seems to float inside like jelly. Both are often faceted, which is unusual for opal, and both should be protected from knocks.

Matrix opal is precious opal spread through its host rock, either ironstone in Queensland boulder matrix or sandstone and quartzite at Andamooka. Andamooka matrix is often treated with sugar and acid to blacken the porous rock so the colour stands out. Matrix opal is cut and polished whole, rock and opal together.

Opalised fossils are shells, bones and wood whose original material has been replaced by opal, found mainly in Australia at Coober Pedy, Andamooka and Lightning Ridge, including opalised shells, belemnites and dinosaur bones. The iridescent shell sold as ammonite opal is usually ammolite, a different gem made of aragonite.