Opal: The Gemstone of Shifting Colours

Opal is unlike any other gemstone. Where rubies and sapphires derive their beauty from a single, consistent colour, opal mesmerises with its play of colour, a dazzling display of shifting spectral hues that dance across the stone as it moves in the light. This phenomenon, unique to opal among gemstones, has captivated humanity for millennia. As the October birthstone, opal holds special meaning for autumn birthdays.

Opal Properties and Characteristics

Opal is a hydrated form of silica (SiO₂·nH₂O) with a water content typically between 3% and 21%. Unlike most gemstones, opal is amorphous, meaning it lacks a crystalline structure. Instead, it consists of microscopic spheres of silica arranged in a regular pattern. When these spheres are uniform in size and arrangement, they diffract light to produce the famous play of colour.

Understanding Play of Colour

Play of colour is the defining characteristic of precious opal. It occurs when white light enters the stone and is diffracted by the orderly array of silica spheres, splitting into its component spectral colours. The colours visible depend on the size of the spheres: smaller spheres produce blues and greens, while larger spheres produce the full spectrum including the prized reds and oranges.

• Broad flash – Large, sweeping areas of colour that shift as the stone moves. Highly valued for its dramatic effect.
• Rolling flash – Colour that appears to roll across the surface of the stone. Very attractive and popular.
• Pinfire – Tiny, closely-spaced points of colour. Less valuable than broad flash but can be very beautiful.
• Harlequin – A rare pattern of angular, mosaic-like patches of colour. The most valuable pattern in opal.

Types of Opal

Black Opal

Black opal is the most valuable variety, found almost exclusively in Lightning Ridge, New South Wales, Australia. The dark body colour (which can range from dark grey to jet black) provides a dramatic backdrop that makes the play of colour appear more vivid and intense. Fine black opals with strong red-on-black play of colour can command prices exceeding those of many precious gemstones.

White (Light) Opal

White opal has a light or milky body colour and is the most common variety. Major sources include Coober Pedy and Mintabie in South Australia. While generally less valuable than black opal, fine white opals with strong play of colour can be very attractive and are more accessible in price.

Boulder Opal

Boulder opal forms as thin seams of precious opal within ironstone matrix. It is found primarily in Queensland, Australia. The natural ironstone backing gives boulder opal a dark background similar to black opal, enhancing the play of colour. Each boulder opal is unique in shape, making these stones particularly interesting for artistic jewellery designs.

Crystal Opal

Crystal opal is transparent to semi-transparent, allowing light to pass through the stone and create a uniquely luminous play of colour. Fine crystal opal can be among the most beautiful opals available.

Fire Opal

Fire opal, primarily from Mexico, is characterised by its vivid body colour of yellow, orange, or red rather than play of colour. Some fire opals do display play of colour, and these are especially prized. Fire opals are often faceted, unlike most other opals which are cut as cabochons.

Ethiopian Opal

Since 2008, Ethiopia has emerged as a significant source of gem-quality opal. Ethiopian opals are typically of the hydrophane variety, meaning they absorb water and may temporarily change appearance when wet. They offer excellent value and often display vivid play of colour.

Opal Sources

• Australia – Produces approximately 90% of the world's precious opal. Lightning Ridge (black opal), Coober Pedy (white opal), and Queensland (boulder opal) are the key areas.
• Ethiopia – An increasingly important source producing vibrant, affordable opal in white, crystal, and occasionally dark body tones.
• Mexico – The classic source of fire opal and some precious opal.
• Brazil, Honduras, Indonesia – Additional sources of varying quality.

Opal Treatments and Assembled Stones

• Smoke treatment – Some light opals are treated with smoke or sugar and acid to darken their body colour, simulating the appearance of black opal. This treatment should be disclosed.
• Doublets – A thin slice of precious opal cemented to a dark backing (often ironstone or black glass). Doublets are significantly less valuable than solid opals but offer an affordable way to enjoy opal's beauty.
• Triplets – A doublet with an additional clear quartz or glass cap on top. Triplets are the least expensive option and are widely used in fashion jewellery.
• Polymer impregnation – Some Ethiopian opals are stabilised with polymer to reduce their tendency to absorb water and crack.

Opal Pricing

• White opal (moderate play of colour): $10 – $100 per carat
• Fine white opal: $100 – $500 per carat
• Boulder opal: $50 – $1,000+ per carat
• Crystal opal: $50 – $3,000 per carat
• Black opal (moderate): $200 – $2,000 per carat
• Fine black opal: $5,000 – $20,000+ per carat
• Exceptional black opal: $50,000+ per carat at auction

Buying Tips for Opal

• Evaluate play of colour first – The brightness, pattern, and range of colours are the primary value factors. Red is the rarest and most valuable colour in play of colour.
• Check from multiple angles – Quality opals should show play of colour from many viewing angles, not just one.
• Ask whether it is solid – Always confirm whether an opal is solid, a doublet, or a triplet. This dramatically affects value.
• Consider body tone – Darker body tones generally command higher prices because they showcase play of colour more effectively.
• Choose protective settings – Opals are relatively soft (5.5-6.5). Bezel settings offer the best protection for rings. See our jewellery types guide for setting options.

Opal Care

Opals require more careful handling than most other gemstones due to their water content and relatively low hardness. Avoid ultrasonic cleaners, steam cleaners, and harsh chemicals. Clean with a damp cloth only. Store opals away from direct heat and sunlight, and avoid extreme temperature changes. Opal doublets and triplets should never be soaked in water, as this can damage the adhesive. For comprehensive care instructions, see our jewellery care guide.

Conclusion

Opal is a truly unique gemstone that offers a visual experience no other stone can match. Whether you choose a fiery black opal from Lightning Ridge, a luminous crystal opal, or a vivid Ethiopian stone, the interplay of shifting colours ensures that every opal is one of a kind. While opals require slightly more care than harder gemstones, their extraordinary beauty more than rewards the effort.