Opal: The Complete Guide

No gem captures the imagination quite like opal. Its play of colour — that shifting, dancing display of spectral hues — has no equivalent in the mineral kingdom. Roman Senator Nonius reportedly chose exile over surrendering his opal to Mark Antony. Queen Victoria distributed opals as gifts throughout European royal families. Today, Australia’s Lightning Ridge produces black opals that sell for prices rivalling fine rubies. Opal is ancient, extraordinary, and full of commercial potential for jewellery professionals who understand it properly.

What Opal Is: Science and Structure

Opal is a mineraloid (not a true mineral in the crystallographic sense) composed of silica (SiO2) with a water content ranging from 3% to 21%. Unlike minerals with a defined crystal structure, opal is amorphous — its silica exists as a gel-like network of hydrated silica spheres rather than an ordered crystal lattice. This unique structure is responsible for both opal’s visual beauty and its vulnerability.

Precious opal (the play-of-colour variety) owes its fire to a highly ordered three-dimensional array of silica spheres of uniform size, arranged in a close-packed grid. When the spacing between spheres is comparable to the wavelength of visible light, diffraction occurs: white light is broken into its spectral components, creating the play of colour. The size of the spheres determines which colours are produced: smaller spheres (around 150 nm) produce violet and blue; larger spheres (around 330 nm) produce red. This is why red in the play of colour is rare and valuable — large, uniform spheres of the right size are harder to form.

Types of Opal

Black opal: dark body colour (dark grey to black), maximum contrast for play of colour — most valuable type

White/light opal: light body colour, play of colour less vivid against pale background — most abundant type

Crystal opal: transparent to semi-transparent body, play of colour visible from multiple directions

Boulder opal: precious opal in thin layers within ironstone matrix — unique Australian type

Fire opal: Mexican origin, orange to red body colour, may or may not show play of colour

Common opal: no play of colour, includes potch (matrix opal) and various coloured opaque varieties

Major Sources

Australia: The Opal Capital

Australia produces approximately 95% of the world’s precious opal. The major deposits are Lightning Ridge (New South Wales) for black opal; Coober Pedy (South Australia) for white opal — the world’s largest opal mining town; Andamooka (South Australia) for crystal and white opal; and Queensland for boulder opal. Lightning Ridge black opal is the benchmark for the entire opal market: the combination of dark body colour and vivid play of colour produces the most dramatic and commercially valuable opal known.

Ethiopia: The New Force

Ethiopian opal from the Welo region (discovered commercially around 2008) has transformed the opal market. Welo opals show play of colour in a hydrophane opal — a porous variety that absorbs water and becomes transparent when wet, which affects its appearance and requires specific care and disclosure. Ethiopian opal is often strikingly beautiful and considerably more affordable than Australian material, creating a new category for the mid-market.

Mexico and Other Sources

Mexican fire opal from the Queretaro region is a distinctive category: orange to red body colour with or without play of colour. Fire opal without play of colour is often faceted to maximise the body colour; fire opal with play of colour is cabochoned. Honduras, Peru, and Brazil also produce opal in smaller commercial quantities.

Quality Factors

Opal quality is assessed on body tone, play of colour brightness, pattern, colour range, and coverage. Body tone (black vs. white) is the primary value determinant for Australian opal. Play of colour brightness is the dominant quality factor: a brilliant, vivid play of colour in a black opal is among the most spectacular visual effects in the gem world.

Pattern names describe specific play of colour arrangements: harlequin (large angular colour patches) is the rarest and most valuable pattern; rolling flash (large sweeping areas of colour) is highly desirable; pinfire (small bright dots) is common in fine stones; broadflash covers large areas. The range of colours present matters: full spectral range including red is more valuable than blue-green only.

Opal Treatments and Constructions

Many commercial opal products are enhanced or assembled. Doublets consist of a thin slice of precious opal cemented to a dark backing material (typically ironstone or black glass) to improve the appearance of thin opal slices. Triplets add a clear quartz or glass cap over the opal layer, providing protection and improving clarity. Both doublets and triplets require disclosure and are sold at a fraction of the price of solid natural opal.

Opal smoking and backing with black epoxy are treatments designed to darken the body colour of light opal artificially. Hydrophane opal should be disclosed as such due to its behaviour when in contact with water, oils, or cleaning solutions.

Durability and Care

Opal is relatively soft (Mohs 5.5-6.5) and moderately brittle. Its water content makes it susceptible to dehydration (crazing — the development of internal fractures) if stored in very dry conditions. Opal should not be cleaned in ultrasonic cleaners, exposed to strong heat or direct sun for extended periods, or cleaned with chemical solvents. Warm water and mild soap with a soft cloth is the recommended cleaning method.