Opal

Extended geology

Opal is amorphous hydrated silica with chemical formula SiO₂·nH₂O, where n can range from 3 to 21 percent by weight. The silica forms as microscopic spheres that stack together to form the stone; in precious opal, these spheres are regular enough in size and arrangement to diffract light, producing play-of-color. In common opal, sphere arrangement is less regular, so the stone shows body color but no spectral fire.

Specific gravity runs 1.98 to 2.25, notably light. Mohs hardness 5.5 to 6.5. Luster vitreous to resinous. No cleavage; conchoidal to uneven fracture. Opal forms in both sedimentary and volcanic settings when silica-rich water precipitates in voids in rock (often volcanic ash, weathered sandstone, or fossilized wood). Formation typically takes very long periods; significant opal deposits are millions of years old.

Extended sourcing

Australia has dominated commercial opal production since the late 1800s. Coober Pedy in South Australia is the world's largest opal-producing town, known for White Opal and Crystal Opal. Mintabie (also in South Australia, mine closed 2019) produced Black Opal and White Opal. Lightning Ridge in New South Wales is the primary source of Black Opal, the most valuable opal variety. Queensland produces Boulder Opal.

Ethiopia's Welo region entered the global market in the 2010s with significant volumes of Welo Opal (hydrophane precious opal). Mexico's Queretaro and Jalisco produce Fire Opal (orange-red translucent, usually without play-of-color). Peru produces Pink Opal and Andean Blue Opal from Andean deposits. The United States (Nevada, Oregon, Idaho), Brazil, Indonesia, and Honduras produce additional commercial material.

Authentication and warning signs

Real opal shows characteristics of its specific variety. Precious opal has play-of-color that moves with viewing angle and emerges from within the stone. Common opal has solid body color with partial translucency. Hardness 5.5 to 6.5 distinguishes opal from harder gemstones, though not from glass, which is about as hard.

Opalite is a glass imitation that's uniformly milky-blue without true play-of-color; it's sold under its own name or sometimes misrepresented as opal. Synthetic opal shows perfect geometric color patches. Doublets (thin real opal on dark backing) and triplets (real opal between dark backing and clear cap) are legitimate but should be disclosed. Dyed and resin-stabilized opal is common, particularly for Ethiopian and common opal varieties.

Historical and cultural context

Pliny the Elder wrote that opal contained the gentler fire of ruby, the glorious purple of amethyst, and the sea-green of emerald, all shining together in incredible union. Roman nobility valued opal highly, and it remained a prestige gem through medieval Europe. The stone features in various royal treasures and church ornaments.

Australian Aboriginal traditions associate opal fields with creation stories: in one tradition, the Creator came down to Earth on a rainbow to bring peace, and where the feet touched the ground, the stones came alive and began to sparkle. The Creator's rainbow bridge left traces in the opal fields. Ethiopian opal use goes back to pre-modern times but became globally significant only recently. In contemporary practice, opal is grouped with perceptual and intuitive stones, with specific varieties carrying more specific associations.

Varieties and trade names

Precious opal varieties (show play-of-color):

White Opal: white or light body with color play, most common precious type.

Black Opal: dark body with vivid color play, most valuable precious opal.

Crystal Opal: transparent to translucent body with color play.

Boulder Opal: opal attached to dark host rock.

Fire Opal: orange-red translucent opal, usually without color play, Mexican.

Welo Opal: Ethiopian hydrophane opal.

Common opal varieties (no play-of-color):

Pink Opal: Peruvian pink common opal.

Blue Opal: Andean blue common opal.

Green Opal: Prase or chrysoprase-colored common opal.

Dendritic Opal: opal with manganese dendrite inclusions.

Hyalite Opal: water-clear, sometimes UV-fluorescent, common opal.

Pricing reality

Common opal (Pink, Blue, Green) is the entry point, tumbled or polished. Fire Opal, Ethiopian Welo Opal, and Australian White Opal are valued by the carat, with Fire Opal rising on color and clarity and Welo and White Opal sitting higher. Fine Black Opal commands the highest prices of any variety, and museum-grade stones reach extraordinary sums.

Value drivers: play-of-color quality and pattern (for precious varieties), body tone, variety-specific rarity, size, origin, and treatment status. Warning signs: 'opal' with uniform milky-blue translucency and no true play-of-color (likely opalite glass), no variety or origin named, or play-of-color that looks too regular (possibly synthetic).