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Crystal Opal
| Hardness | 5.5-6 |
| Specific gravity | 1.98-2.20 |
| Refractive index | 1.37-1.47 |
| Crystal system | 非晶質（含水シリカ） |
Body colors from completely water-clear through pale grey, milky, and faintly amber, with play-of-color spanning every spectral hue depending on the silica-sphere diameter. The most prized stones show broad flashes of red, orange, and green against an almost invisible body.
- Aligned silica spheres responsible for play-of-color (visible only under SEM)
- nclusions throughout the host
- Occasional sand or potch matrix relics near the edges
- Crazing — fine surface fracturing in unstable or improperly stored material
- Amorphous, singly refractive (no )
- Play-of-color from diffraction by ordered silica spheres
- Vitreous to resinous luster
- Transparent to translucent body
- 01Transparent or translucent body — the key differentiator from white opal's milky body
- 02Play-of-color suspended inside the stone rather than floating on the surface
- 03Hydrophane behavior (color shift when wet) indicates Ethiopian origin
- 04Inspect the side profile for the dark backing layer of a doublet or the glass cap of a triplet
- Mohs 5.5–6.5 — softer than most gems; protective settings recommended
- Avoid sudden temperature changes — opal contains structural water and can craze
- Never use ultrasonic or steam cleaners
- Hydrophane Welo material should be kept away from water, oils, perfume, and cosmetics; store flat in stable humidity
Roughly $20–$200/ct for commercial Ethiopian crystal opal with average play-of-color, $300–$1,500/ct for fine Coober Pedy material with broad red and green flashes, and over $5,000/ct for top Lightning Ridge crystal opal showing harlequin or red-on-red patterns. Pattern, brightness, and body transparency drive value more than carat weight alone.
Note: Australian crystal opal is essentially untreated and stable. Ethiopian crystal opal is almost always hydrophane and will temporarily change appearance when wet — disclosure is increasingly expected at the wholesale level. Doublets (thin slice over potch) and triplets (with a glass cap) exist, so always inspect the side profile under magnification.
Crystal opal is hydrated amorphous silica (SiO₂·nH₂O) with the same submicroscopic silica-sphere lattice that produces play-of-color in all precious opal, but with a body clear enough that the diffraction colors appear suspended inside the stone. The term applies whenever the body is transparent or noticeably translucent — 'crystal' here refers to optical clarity, not crystal structure. Coober Pedy and Lightning Ridge in Australia, and Welo in Ethiopia, dominate global supply.
Origins
Australia is the historical heartland: Coober Pedy (South Australia, the official Australian opal capital since 1987, with mining since 1915), Lightning Ridge (New South Wales, also the black opal source), and White Cliffs (the first major Australian opal field, opened 1889). Mexican white-body crystal opal comes from Querétaro. Ethiopia's Wollo Province has produced large quantities of hydrophane crystal opal since 2008 — beautiful but moisture-sensitive (see welo_opal). Brazil's Piauí state supplies smaller volumes.
History
Crystal opal was differentiated from black and white opal in the early Australian trade of the 1900s–1920s, when miners and cutters needed shorthand for the new transparent variety pouring out of Coober Pedy after the 1915 strike by Willie Hutchison's prospecting party. Pliny the Elder's description of opal in Naturalis Historia (~77 CE) — 'in one stone the gentle fire of the ruby, the rich purple of the amethyst, the sea-green of the emerald, glittering together in an inconceivable mixture of light' — fits crystal opal as well as any of its cousins.
Lore & symbolism
The October birthstone, symbolizing hope, creativity, and pure inspiration. Crystal opal in particular is called 'the stone of inner light' in modern lapidary writing — the play-of-color appearing to glow from within. The 14th wedding anniversary gem.
Tools to confirm this stone
Tools that help confirm Crystal Opal. Tap any item to jump to the matching section on the gem tools page.
- 最終確認日
- 2026年4月28日
- 参 考 文 献
- Gem Encyclopedia／ GIA (Gemological Institute of America)
- 宝石鑑別基準／ 中央宝石研究所 (CGL)
- Mineral & Gem Database／ Mindat.org / Gemdat.org
- 宝石学入門／ 全国宝石学協会
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