What is opal? The stone that contains every colour at once

Play-of-colour: the diffraction physics

Precious opal's play-of-colour is produced by diffraction, not by pigment or absorption. When white light enters a precious opal, it encounters the regular array of silica spheres stacked in the opal's structure. The spacing between the spheres is comparable to the wavelengths of visible light (approximately 380–700 nanometres). When the inter-sphere spacing matches specific light wavelengths, those wavelengths are diffracted, selectively reinforced in specific directions by constructive interference, while others are cancelled. The observer sees the reinforced wavelength as a vivid spectral colour (Nassau, K., 1978; GIA Gem Reference Guide, 2006, pp. 62–63).

The colour produced depends on sphere size: smaller spheres (approximately 150 nm spacing) diffract violet and blue; medium spheres (approximately 200 nm) diffract green and yellow; larger spheres (approximately 250–300 nm) diffract red and orange. The finest black opals from Lightning Ridge contain a distribution of sphere sizes that diffracts across the full spectral range, producing the full rainbow of colour visible in the finest specimens. The term "play-of-colour" in gemology refers specifically to this spectral colour display from diffraction, distinguished from the iridescence of interference colours in other materials (GIA; Nassau, 1978).

Opal play-of-colour physics. Common opal has irregular silica sphere sizes, no regular spacing means no constructive diffraction, producing a white or grey appearance. Precious opal has uniform spheres in a regular array that diffracts specific wavelengths based on sphere spacing. Mixed sphere sizes produce full-spectrum play. Source: Nassau (1978); GIA (2006).

The five commercially recognised opal types

The opal industry and GIA recognise five primary commercial types defined by their body colour (the background colour of the stone independent of play-of-colour) and geological form:

Black opal: Dark body colour (black, dark grey, dark blue) with play-of-colour. The dark background makes the spectral colours appear more vivid by contrast. The finest black opal comes from Lightning Ridge, New South Wales, Australia. Black opal commands the highest per-carat prices of any opal type.

White opal (light opal): Light or white body colour with play-of-colour. Less vivid apparent play than black opal because the light background reduces colour contrast. Coober Pedy, South Australia, is the primary source. More common and more affordable than black opal.

Crystal opal: Transparent to semi-transparent body with play-of-colour. The transparency allows play-of-colour to be seen from depth within the stone, producing a three-dimensional quality. Can have any body colour from colourless to dark; transparency is the defining characteristic.

Boulder opal: Australian opal attached to its ironstone host rock (matrix), cut with the ironstone as a natural backing. The dark ironstone acts as a natural back for the opal layer, producing a stone with the dark background effect of black opal but in thinner layers than would be possible as a solid stone.

Fire opal (Mexican fire opal): Transparent to translucent orange to red opal from Mexico, most commonly not showing play-of-colour. The colour comes from iron oxide impurities rather than play-of-colour. Some Mexican fire opal does show play-of-colour; the best examples are exceptionally vivid. Mexico's Querétaro and Guerrero states are the primary sources.

Black opal from Lightning Ridge

Lightning Ridge in northern New South Wales is the world's most famous opal field and the exclusive source of the finest black opal. The town is approximately 770 kilometres northwest of Sydney, in semi-arid country that was once the floor of an inland sea. The opal forms in sedimentary sandstone sequences from the Cretaceous period, approximately 100 million years old. The specific geology that produces black opal: organic-rich sediment levels within the sandstone provided the silica-mobilising chemical environment; the dark ironstone and carbon in the host rock contributes to the dark body colour that makes Lightning Ridge black opal distinct from light opal (GIA; Wise, 2016, pp. 201–205).

The finest Lightning Ridge black opals achieve play-of-colour across the full spectrum with vivid saturation against a black or very dark background. A stone showing vivid red play on a black body is considered the finest possible combination: red is the highest-energy diffracted colour (produced by the largest sphere spacing), and its rarity against a black background drives the highest prices. A fine Lightning Ridge black opal with vivid red play above 5 carats can command USD 10,000–50,000 per carat at specialist auction (GIA; Wise, 2016; Christie's).

White opal from Coober Pedy

Coober Pedy in South Australia is the world's largest commercial opal producer by volume, supplying the majority of the world's white opal. The town is famous for its underground dugout homes, built below the desert surface to escape summer temperatures that reach 50°C. The opal forms in sandy Cretaceous sediments at shallow depths, making it more accessible than the Lightning Ridge black opal which occurs in harder sandstone (GIA; Wise, 2016, pp. 205–208).

White opal is more affordable and more available than black opal and constitutes the majority of opal jewellery at commercial price points. The play-of-colour in fine white opal can be vivid and attractive; the limitation relative to black opal is the pale background that reduces the apparent brilliance of the colour display.

Boulder opal from Queensland

Boulder opal forms in cracks and fissures within ironstone boulders in the Queensland outback, particularly in Winton, Quilpie, and Yowah areas. The opal fills the fissures in the ironstone host, and the stones are cut to include a natural backing of the dark ironstone, which gives boulder opal the dark background effect of black opal at a more accessible price. Some boulder opal cutters produce "matrix opal", stones where opal fills small pores throughout the ironstone host, and "Yowah nuts", concretions with an opal centre. Queensland is also the source of "Koroit opal," a highly patterned boulder opal variety with distinctive swirling colour patterns (GIA; Wise, 2016, pp. 208–210).

Mexican fire opal

Mexican fire opal is the only major commercially significant opal not from Australia. The transparent to translucent orange-red material from Querétaro and Guerrero states in Mexico is coloured by iron oxide rather than play-of-colour diffraction. The finest Mexican fire opal shows a vivid, saturated orange or red-orange that is striking on its own; the rarer stones that also show play-of-colour are among the most spectacular opal specimens known. Mexican fire opal is typically faceted rather than cabochon-cut, unlike most precious opal (GIA; Wise, 2016, pp. 210–212).

Ethiopian opal: the newer source

Ethiopian opal, discovered commercially in the Wollo Province (also called Welo) in 2008, rapidly became significant in the opal market due to its vivid play-of-colour, crystal transparency, and accessibility of supply. Ethiopian opal presents one specific challenge not common in Australian material: hydrophane character. Hydrophane opal has micro-porous structure that absorbs water, causing the stone to temporarily lose some or all of its play-of-colour when wet, and to regain it when dry. This characteristic, while not permanently damaging, surprises buyers who do not expect it. GIA includes hydrophane character in its Ethiopian opal reports (GIA; Wise, 2016).

Care challenges unique to opal

Opal requires more careful handling than most other gem minerals due to three specific characteristics:

Low hardness: Mohs 5.5–6.5, significantly softer than corundum (9), beryl (7.5–8), and quartz (7). Opal surfaces scratch easily in normal wear environments. Daily wear rings require protective settings (bezel or half-bezel) and should be worn with awareness of scratch risk.

Water content: Gem opal contains 3–21% water. If opal dries out in very low humidity environments over extended periods, it can develop internal cracking (crazing). Avoiding prolonged storage in very low humidity, and not exposing opal to sudden temperature changes, protects against crazing. Ultrasonic cleaning and steam cleaning are not appropriate for opal.

Thermal shock sensitivity: Rapid temperature change can cause cracking. Never use steam cleaners or expose fine opal to sudden heat. Clean only with warm water and a soft cloth (GIA; Wise, 2016).