Blue opal is a variety of common opal — that is, opal lacking the iridescent play-of-colour that characterises precious opal — distinguished by a translucent to opaque body colour ranging from pale sky blue and seafoam to a deeper teal. It is valued not for spectral fire but for the quiet, even beauty of its colour and the gentle waxy to resinous lustre that makes well-cut cabochons appear almost luminous. The two principal sources are the Andean highlands of Peru, which supply the majority of material reaching the international market, and the state of Oregon in the United States, whose deposits yield a rarer, often more saturated blue. Both varieties share the fundamental chemistry of opal — hydrated silicon dioxide (SiO₂·nH₂O) — but differ in geological context, colour character, and trade significance.

Mineralogy and Physical Properties

Like all opals, blue opal is an amorphous mineraloid rather than a true crystalline mineral. Its internal structure consists of closely packed spheres of silica in a silica-gel matrix; in common opal the spheres are either too irregular or too poorly ordered to diffract visible light into spectral colours, so no play-of-colour results. The blue body colour arises from trace copper impurities — a conclusion supported by spectroscopic studies — combined with the scattering of short-wavelength light by the silica microstructure, an effect analogous in principle to Rayleigh scattering.

• Chemical formula: SiO₂·nH₂O (water content typically 6–10% by weight)
• Hardness (Mohs): 5.5–6
• Specific gravity: approximately 1.98–2.20 (variable with water content)
• Refractive index: approximately 1.37–1.47 (singly refractive, isotropic)
• Lustre: waxy to resinous
• Transparency: translucent to opaque; the finest Peruvian material is semi-translucent
• Fracture: conchoidal

The relatively high water content that gives opal its distinctive optical character is also its principal vulnerability. Blue opal is susceptible to dehydration in low-humidity environments, which can cause surface crazing — a network of fine cracks — and a dulling of colour. This property demands careful handling, storage away from prolonged heat or direct sunlight, and avoidance of ultrasonic or steam cleaning.

Peruvian (Andean) Blue Opal

The most commercially significant blue opal originates from deposits in the Andes of Peru, principally in the departments of Ica and Lima, with the Acari district historically among the better-documented localities. Peruvian blue opal — often called Andean opal in the trade — occurs in volcanic and sedimentary sequences associated with Tertiary-age andesitic volcanism, typically filling veins and nodular masses within the host rock.

Its colour is characteristically a soft, pastel blue — sometimes described as the blue of a clear Andean sky — with a gentle blue-green or aqua cast in some specimens. The finest pieces exhibit a pleasing semi-translucency that allows light to pass through thin sections, lending carved and faceted pieces a delicate, almost ethereal quality. Dendritic inclusions of manganese oxide are occasionally present, creating natural landscape-like patterns that are considered decorative rather than detrimental in the carving trade.

Peru has been a source of this material since pre-Columbian times; the Inca and earlier Andean cultures used the stone for ornamental and ritual objects. In the modern gem trade, Peruvian blue opal is cut primarily as cabochons, beads, and carved pieces. It is also faceted — a less common treatment that exploits the stone's translucency — though faceting requires care given the material's moderate hardness and sensitivity to stress. The colour is generally stable under normal wearing conditions, but prolonged exposure to very dry conditions or sudden temperature changes should be avoided.

Oregon Blue Opal

Oregon produces blue opal from volcanic deposits in the central and eastern parts of the state, with material associated with the region's extensive Tertiary basaltic and rhyolitic volcanism. Oregon blue opal tends toward a more saturated, sometimes electric blue compared with the pastel tones of Peruvian material, and fine specimens can approach the richness of fine aquamarine in hue, though the body colour remains distinctly different in character.

The material is considerably rarer than its Peruvian counterpart and commands correspondingly higher prices per carat when of gem quality. Oregon blue opal is found in association with other opal varieties from the state, including the celebrated Oregon fire opal and the precious opal deposits of the Virgin Valley. Because production is limited and inconsistent, Oregon blue opal occupies a collector-oriented niche rather than a mainstream commercial position.

Other Sources

Blue opal of varying quality has been reported from additional localities, including deposits in Tanzania, Indonesia, and Australia, though none of these approach Peru or Oregon in terms of consistent gem-quality output or market recognition. Australian blue opal, where it occurs, is often associated with the broader opal-producing regions of New South Wales and South Australia, but the material is generally distinguished from the country's celebrated precious opal and does not dominate export statistics.

Treatments and Enhancements

Blue opal, like many common opals, may be subjected to impregnation with resins or polymers to improve surface stability, reduce porosity, and enhance colour saturation. This treatment is not always disclosed in the lower end of the market, and buyers of significant pieces are advised to request a laboratory report from a recognised gemmological laboratory. Dyeing is also possible, given opal's porous microstructure; dyed material typically shows uneven colour concentration under magnification, particularly along fractures and grain boundaries. Some Peruvian material is sold after stabilisation treatments that are broadly analogous to those applied to turquoise.

Reputable laboratories including the GIA Gem Trade Laboratory and Gübelin Gem Lab are equipped to identify resin impregnation and dyeing in opal. For significant purchases, independent testing is prudent.

Gem Cutting and Use

The cabochon is the dominant cut for blue opal, allowing the cutter to maximise colour display and translucency while minimising the risk of fracture. A well-proportioned cabochon with a smooth, polished dome will show the characteristic waxy lustre to best advantage. Beads are also widely produced, particularly from Peruvian material, and are used extensively in silver jewellery — a pairing that complements the stone's cool, muted palette. Carving is practised with fine material, and the Peruvian tradition of working the stone into sculptural forms has a long cultural history.

Faceted blue opal, while less common, can be striking when the material is sufficiently translucent. Cutters typically favour shallow pavilion angles to retain light within the stone and avoid the dark, windowed appearance that can result from overly deep proportions in translucent material.

Care and Handling

Given the moderate hardness and sensitivity to dehydration, blue opal jewellery requires more considered care than harder gem varieties. Recommended precautions include:

• Storage in a slightly humid environment, or with a damp cloth in a sealed container, if the piece will not be worn for extended periods
• Avoidance of prolonged exposure to direct sunlight, heat sources, or very dry air
• Cleaning with a soft, damp cloth only — no ultrasonic cleaners, steam, or harsh chemical solvents
• Protection from sharp impacts, which can cause chipping or fracture along the conchoidal cleavage

In the Trade

Peruvian blue opal is widely available through gem dealers and is a staple of the silver jewellery market, particularly in South American artisan traditions and in the broader international market for affordable semi-precious stones. Pricing is driven primarily by colour quality — the most desirable stones showing an even, medium-intensity blue without excessive green or grey modifiers — and by translucency, with semi-translucent material commanding premiums over fully opaque pieces. Oregon blue opal, being scarcer, is traded more as a collector's stone and is less frequently encountered in commercial jewellery settings.

Neither variety should be confused with blue precious opal, which displays play-of-colour and is valued by an entirely different set of criteria. The distinction is straightforward in person but occasionally obscured in online listings where photography can exaggerate or misrepresent optical effects.

Further Reading

• GIA Opal Description and Buying Guide — gia.edu
• International Gem Society: Opal Gem Information — gemsociety.org