Cut gemstone
Rough crystal
Opal is a unique mineraloid — an amorphous form of hydrated silicon dioxide (SiO₂·nH₂O). Its unique property is "play of color" — an optical phenomenon where the stone displays a rainbow of moving colors. It forms by the sedimentation of colloidal silica spheres into rock cavities, which then diffract light.
Colors
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Shapes & cuts
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Jewelry suitability
Origins
Australia (Lightning Ridge, Coober Pedy, Andamooka) · Ethiopia (Welo) · Mexico (Jalisco, Querétaro) · Brazil
History
Pliny the Elder wrote in Naturalis Historia (75 AD): "Some opals carry such a play within them that they equal the deepest and richest colors of painters." He named it "Pandora," "Light of the World," and "Empress." In ancient Rome, opalus was synonymous with "precious stone."
Opal was so prized by the Romans in the 1st century that Senator Marc Antony allegedly drove Senator Nonius from Rome because he refused to sell an opal valued at 2 million sesterces. Nonius chose exile rather than surrender the stone.
The opal superstition: Walter Scott's novel "Anne of Geierstein" (1829) sparked a false belief that opal is unlucky — Lady Hermione had an opal that changed color and crumbled on contact with holy water. Opal sales reportedly fell 50% in five years after the book's publication.
In depth
Physics of play of color: Opal consists of regularly arranged microscopic silica spheres (150–400 nm). Light interferes and diffracts between spheres, splitting into the color spectrum. Sphere size determines color: smaller → blue/violet; larger → red/orange. Opals with red in play of color are rarest and most valuable.
Black opal (Lightning Ridge): The most valuable variety — dark body tone dramatically enhances color contrast in play of color. Lightning Ridge, New South Wales, Australia, is the only source of high-quality true black opals. Prices reach record values; "Aurora Australis" (180 ct) was valued at AUD 1 million.
Crystal opal: Transparent to semi-transparent; play of color visible throughout the stone. Coober Pedy and Andamooka are primary sources.
White opal: Milky white body; most common and accessible. Play of color is less contrasting than black.
Ethiopian opal (Welo): Discovered in 2008. Often crystal to semi-transparent with exceptionally intense play of color. Special property: hydration — in water they become more transparent ("hydrophane"). Cheaper than Australian, often excellent value.
Fire opal (Mexico): Orange to red body color WITHOUT play of color (or minimal). Its value is based on body color, not play. It is the only opal cut in faceted (not cabochon) shape.
Water content: Opal contains 3–21% water in its structure. Drying can cause cracking ("crazing") — one of the risks with this stone.
Rarity
A high-quality black opal with intense play-of-color is one of the rarest and most valuable gemstones in the world. Lightning Ridge in New South Wales, Australia, is the only deposit of fine-quality black opal — it is mined by hand by small independent miners.
Boulder opal from Queensland, Australia, is also premium and unique — opal grows in veins along dark ironstone, providing a natural contrasting background.
Ethiopian opal (Welo, discovered 2008) has changed the market: it is more accessible and often shows vivid play-of-color, but is less stable (absorbs water — "hydrophane opal").
Did you know
Opal is not a mineral in the classical sense — it is an amorphous form of silica (SiO₂·nH₂O). It contains 3–20% water bound in a microscopic lattice of silica spheres with diameter 150–400 nanometers. When light hits this lattice, it diffracts and disperses into the color spectrum — this is play-of-color.
No two opals are identical — each has a unique play-of-color. This results from the random orientation of silica spheres during crystallization. Even the same opal shows different colors when rotated.
Australia provides ~95% of world opal production. However, while production volume is large, quality stones (especially black opal from Lightning Ridge) are exceptionally rare.
Natural vs lab-grown
Synthetic opal has been commercially available since the 1970s. Key producers:
- Gilson opal (Pierre Gilson, France, 1974) — first commercial synthetic opal; produced by sedimentation of synthetic silica spheres; color play is exceptionally intense
- Kyocera (Japan) — produces synthetic opal with play of color
GIA distinguishes synthetic opal from natural via microscopy: synthetic opal has a characteristic "columnar" or "lattice" sphere pattern absent in natural opal. Natural opal has a more irregular, "organic" sphere arrangement.
Imitations (not synthetic opal, but imitants):
- Opal doublet — a thin slice of real opal glued to a dark backing (onyx, glass) to imitate black opal; lower value than solid opal
- Opal triplet — doublet with an additional crystal cap layer for protection
- Laboratory-colored glass with color play
For buyers: GIA certificate distinguishes natural from synthetic. Solid opal is premium; doublets and triplets must be disclosed and are significantly cheaper.
Treatment
Opals are frequently impregnated with plastic (Opticon) or oil to improve stability and reduce fracture visibility. Smoke treatment (smoking) artificially darkens the body tone to imitate black opal. Doublets (thin opal on dark backing) and triplets (doublet with crystal cap) are legitimate but cheaper alternatives that must be disclosed.
Optical phenomena
Play of Color
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Play of color is a spectacular optical phenomenon unique to opals, where the stone displays vivid spectral colors — reds, greens, blues, yellows, and violets — that shift and move as the stone or light source is rotated. Fine opals from Coober Pedy in Australia display the full spectral range with red, which is the rarest and most valuable color. Mexican fire opal has a fiery orange body color without play of color.
Fluorescence
Fluorescenca
Fluorescence is a phenomenon where a gemstone absorbs invisible UV light (ultraviolet) and immediately re-emits it as visible light of a different color. Burmese rubies fluoresce vivid red under UV light — this dramatically enhances their appearance and value. Many diamonds fluoresce blue, which some buyers find desirable (gives blue-white appearance in diffused daylight) and others undesirable (gives an "oily" look under incandescent light). Emeralds often fluoresce red.
Care & maintenance
Opal requires special care. Hardness 5.5–6.5 makes it fragile. Water content means: avoid thermal shock, dry environments, and direct sunlight — crazing is irreversible. Never clean with ultrasonic or chemicals. Clean only with a soft damp cloth. Store in a humid environment (with a small water dish or damp cotton in a closed box). Remove before swimming, cleaning, sports.
What to look for when buying