Common opal is the broad category of hydrated amorphous silica (SiO₂·nH₂O) that lacks the play-of-colour — the spectral iridescence — that defines precious opal. Known to miners and lapidaries as potch, it shares the same fundamental chemistry and physical properties as its more celebrated relative but owes its appeal entirely to body colour, translucency, diaphaneity, and, in certain varieties, a waxy to resinous lustre that can be quietly compelling. Far from being a mere by-product of opal mining, common opal encompasses a wide range of gem-quality materials — from the pellucid, water-clear glass of hyalite to the seafoam blue-greens of Andean opal from Peru — that have been fashioned into jewellery, carvings, and collector specimens for centuries. Understanding common opal requires setting aside the expectation of fire and appreciating what these stones genuinely offer on their own terms.

Chemical Composition and Structure

All opal, common or precious, is an amorphous hydrated silica. The water content, expressed as nH₂O, typically ranges from roughly 3 to 21 per cent by weight, though most gem-quality material falls between 6 and 10 per cent. Because opal is amorphous rather than crystalline, it has no cleavage and no true crystal structure; it belongs to the mineraloid class rather than the mineral class in strict nomenclature.

The critical structural distinction between precious and common opal lies at the submicroscopic level. Precious opal contains a three-dimensional lattice of uniformly sized silica spheres — typically 150 to 400 nanometres in diameter — packed in an orderly, close-packed arrangement. This regularity diffracts visible light, producing the spectral play-of-colour known as adularescence in some literature, though the correct term for opal's phenomenon is play-of-colour or opalescence in the strict gemmological sense. Common opal, by contrast, contains silica spheres that are either too small, too large, or — more commonly — insufficiently ordered. Without that periodic three-dimensional grating, no diffraction occurs, and the stone presents only its body colour to the eye. This disordered silica structure is sometimes described as CT opal (cryptocrystalline or poorly ordered cristobalite/tridymite domains) in the petrological literature, distinguishing it from the well-ordered A-opal of gem precious opal.

Physical and Optical Properties

Common opal shares the following properties with all opal:

• Hardness: 5.5 to 6.5 on the Mohs scale, varying with water content and structural coherence.
• Specific gravity: 1.98 to 2.20, again variable with porosity and water content; most gem-quality common opal falls near 2.10.
• Refractive index: 1.37 to 1.47, typically measured as a single reading on the refractometer because opal is isotropic (amorphous). Most common opal reads near 1.42 to 1.45.
• Lustre: Waxy to resinous, occasionally vitreous in hyalite and other glassy varieties.
• Fracture: Conchoidal, with a characteristic smooth curved surface.
• Transparency: Ranges from opaque (most white and grey potch) through translucent (milk opal, many Andean opals) to transparent (hyalite, some blue Peruvian material).
• Fluorescence: Variable. Hyalite from certain localities — notably Valec in the Czech Republic and Zacatecas in Mexico — fluoresces a vivid green under both longwave and shortwave ultraviolet, owing to trace uranium content. Other common opals may show white, cream, or no fluorescence.

Because opal is thermally sensitive and contains structurally bound water, all opal — common and precious alike — is susceptible to crazing (the development of fine surface cracks) if subjected to rapid temperature change, prolonged low humidity, or mechanical shock. This fragility is a practical consideration for cutters and jewellers working with the material.

Principal Varieties

The term "common opal" is an umbrella, and gemmological literature recognises several named varieties distinguished by colour, transparency, or locality.

Potch

In the opal-mining vocabulary of Australia — the world's dominant opal producer — potch refers specifically to the common opal that surrounds, underlies, or overlies seams of precious opal. It is typically white, grey, or black and is ordinarily discarded or used as backing material in doublets and triplets. Black potch is particularly valued as a backing because it enhances the apparent depth and colour saturation of the precious opal layer cemented above it. In this industrial context, potch is not a gem material in its own right but a structural component of composite stones.

Milk Opal

Milk opal — sometimes called white opal in trade contexts, though that term more often denotes precious opal with a white body colour — is a translucent to semi-opaque common opal with a milky, porcelain-like appearance. Body colours range from pure white through cream and pale yellow. Fine milk opal has a gentle luminosity that has made it a modest but consistent presence in cabochon cutting. Deposits occur in Mexico, Honduras, and various Central European localities.

Hyalite

Hyalite (from the Greek hyalos, glass) is a colourless, water-clear variety of common opal that forms as botryoidal (grape-like) or stalactitic crusts on rock surfaces, typically in volcanic settings. Its glassy transparency and rounded, bubble-like morphology make it immediately recognisable. The most notable gem-quality hyalite comes from Valec (formerly Waltsch) in the Karlovy Vary region of the Czech Republic, a locality known since the eighteenth century, and from deposits in Zacatecas and San Luis Potosí, Mexico. The uranium-bearing Mexican and Czech material fluoresces an intense green — sometimes described as almost neon — under ultraviolet light, a property that has made it a favourite among fluorescent-mineral collectors. Hyalite is rarely faceted but is prized as a specimen mineral and occasionally set in contemporary jewellery for its unusual optical character.

Andean Opal (Peruvian Opal)

Among the most commercially significant varieties of common opal, Andean opal — mined primarily in the Ica and Lima regions of Peru — occurs in a distinctive range of blue-green to blue body colours, as well as pink and white. The blue-green material, sometimes compared to the colour of a tropical lagoon or to the sky at high altitude, derives its colour from organic compounds (specifically, quinone-type pigments) rather than from trace metals. This organic colourant is sensitive to prolonged ultraviolet exposure, and some Peruvian opal will fade over time if left in direct sunlight — a fact that should be communicated to buyers. The pink variety, sometimes called rosa opal in the Peruvian trade, ranges from pale blush to a moderately saturated rose and is fashioned extensively into beads, cabochons, and carvings. Andean opal is translucent to semi-transparent and takes a good polish. It has been used in pre-Columbian Andean cultures and remains an important export gemstone for Peru today.

Prase Opal (Chrysopal)

A green variety of common opal coloured by nickel, prase opal (also called chrysopal in older literature) occurs most notably in Tanzania and Poland (the Szklary deposit in Lower Silesia). The Tanzanian material, sometimes marketed as African opal, ranges from apple green to a deeper, slightly yellowish green and is translucent to opaque. It is fashioned into cabochons and beads and occupies a modest but stable position in the coloured-stone trade.

Fire Opal (Body Colour Only)

It is worth noting that fire opal — the orange to red variety from Mexico and, to a lesser extent, Ethiopia and Brazil — straddles the boundary between common and precious opal. Fire opal may or may not display play-of-colour; when it does not, it is technically a common opal valued purely for its vivid orange or red body colour. Mexican fire opal without play-of-colour is frequently faceted, taking advantage of its transparency, and is a well-established gem material in its own right. The GIA and most gemmological authorities treat fire opal as a distinct variety rather than subsuming it entirely under common opal, but the overlap is real and worth acknowledging.

Principal Localities

Common opal is geologically ubiquitous — it forms wherever silica-rich hydrothermal or meteoric waters percolate through rock and deposit silica gel that subsequently solidifies. Gem-quality material, however, is more restricted:

• Australia: Coober Pedy, Lightning Ridge, Mintabie, and Andamooka produce vast quantities of potch as a by-product of precious opal mining. Some Australian common opal, particularly translucent white material, is cut for the domestic market.
• Peru: The Acari and Ica districts are the primary sources of blue-green and pink Andean opal.
• Mexico: Querétaro, Jalisco, and Hidalgo states produce milk opal, hyalite, and the body-colour-only fire opals noted above.
• Czech Republic: Valec (Waltsch) remains the classic European locality for hyalite.
• Tanzania: Prase opal (nickel-green) from the Hanang and Arusha regions.
• Poland: The Szklary nickel deposit in Lower Silesia has produced prase opal since the nineteenth century.
• USA: Oregon (the Owhyee region near Spencer, Idaho, and the Virgin Valley in Nevada) produces common opal alongside precious material. The Virgin Valley is notable for producing black common opal that can be spectacular in specimen form but is notoriously prone to crazing once removed from its host matrix.
• Brazil: Piauí and other states produce white and pink common opal used extensively in bead production.

Treatments and Enhancements

Common opal is subject to several treatments, some widely accepted and some that require disclosure:

• Impregnation with resins or oils: Porous common opal — particularly white and light-coloured material — may be impregnated with colourless resins or oils to improve stability, reduce crazing risk, and enhance translucency. This treatment is not always disclosed and can be detected by gemmological laboratories through infrared spectroscopy (FTIR), which reveals organic absorption bands absent in untreated material.
• Dyeing: White or pale common opal is occasionally dyed to simulate more desirable body colours, including the blue-green of Andean opal. Dyed material typically shows colour concentrations along fractures and may bleed colour when tested with a solvent-dampened swab. Reputable laboratories can identify dyeing through spectroscopic and microscopic examination.
• Smoke treatment: A traditional technique used in Mexico involves wrapping opal in paper and heating it so that carbon from the smoke penetrates the stone, darkening it. This can give pale common opal a grey or black body colour superficially resembling black precious opal. The treatment is generally considered acceptable when disclosed but misrepresentation is a known trade issue.
• Sugar-acid treatment: Similar to the carbon-smoke method, this process involves soaking opal in a sugar solution followed by treatment with sulphuric acid, which carbonises the sugar within the stone's pores and darkens the body colour. Again, disclosure is essential.

Buyers of common opal at significant price points — particularly Andean opal and prase opal — are advised to request laboratory reports from recognised gemmological laboratories such as the GIA, Gübelin Gem Lab, or SSEF, which can confirm natural colour and identify treatments.

Cutting and Use in Jewellery

Common opal is almost universally cut en cabochon, the domed form that best displays translucency and body colour. Andean opal and prase opal are also fashioned into beads, briolettes, and carved forms. Hyalite, being too fragile and irregular in its botryoidal morphology for conventional cutting, is usually set as a specimen or occasionally incorporated into contemporary jewellery in its natural form.

The softness of common opal (5.5 to 6.5 Mohs) means it is unsuitable for rings intended for daily wear unless set in protective bezels or used in pieces that are not subject to abrasion. Pendants, earrings, and brooches are more appropriate settings. Andean opal, with its appealing colour and moderate translucency, has found a consistent market in silver jewellery from Peru and Bolivia, as well as in the broader international coloured-stone trade.

In the Art Nouveau period, the milky translucency of common opal — particularly white and pale blue material — was prized by designers such as René Lalique, who incorporated it into enamel-and-gem compositions where its quiet luminosity complemented rather than competed with the surrounding metalwork. This historical use underscores that common opal has always had a distinct aesthetic identity, separate from and not inferior to precious opal.

Value and Market Context

Common opal occupies a broad price range, from negligible (potch and low-grade white material) to several hundred dollars per carat for fine, deeply coloured Andean opal or exceptional prase opal. The key value factors are:

• Body colour saturation and evenness: The more vivid and uniform the colour, the higher the value. In Andean opal, a deep, evenly distributed blue-green commands the highest prices.
• Translucency: Greater translucency generally increases desirability, as it gives the stone a luminous, almost glowing quality.
• Clarity: Freedom from visible inclusions, fractures, and matrix patches is preferred, though some matrix inclusions are accepted in certain varieties.
• Size: Large, clean pieces of fine Andean opal or prase opal are uncommon and command premiums accordingly.
• Treatment status: Untreated material commands a premium over dyed or impregnated stones, particularly as laboratory testing has become more accessible.

Common opal is not tracked by the major coloured-stone price guides (such as the GemGuide) with the same granularity as ruby, sapphire, or emerald, reflecting its lower average price point and the heterogeneity of the category. Trade pricing is largely negotiated based on the factors above and on current supply from producing regions.

Collector Interest

Beyond the jewellery trade, common opal has a significant following among mineral collectors. Hyalite specimens — particularly those with strong uranium-induced green fluorescence — are eagerly sought. Virgin Valley black opal, despite its crazing tendency, attracts collectors willing to accept the instability for the sake of the material's dramatic appearance. Botryoidal common opal from various localities, with its rounded, lustrous surfaces, is a perennial favourite in the mineral specimen market. The intersection of gemmological and mineralogical interest gives common opal a broader collector base than its jewellery market position alone would suggest.

Further Reading

• GIA: Opal Classification and Nomenclature
• GIA Gems & Gemology: Peruvian Opal
• International Gem Society: Opal
• AGTA: Opal Gemstone Information