Key properties
| Chemical formula | SiO2·nH2O |
| Crystal system | Amorphous |
| Mohs hardness | 5-6.5 |
| Toughness | Very poor to fair |
| Refractive index | 1.370-1.470 |
| Optic character | Singly refractive (anomalous double refraction common from strain) |
| Specific gravity | 1.98-2.23 |
| Cleavage | None |
| Fracture | Conchoidal to uneven |
| Habit | Amorphous masses, veins and nodules in weathered rock; botryoidal and stalactitic forms; thin seams on ironstone (boulder opal) |
| Lustre | Subvitreous to waxy; polish: Vitreous to resinous |
| UV fluorescence | White and black opal: inert to moderate light blue, green or yellow in LW and SW, sometimes phosphorescing; common opal often fluoresces strong green or yellowish green; fire opal inert to moderate greenish brown |
| Pleochroism | None |
| Cause of colour | Play-of-colour is diffraction from regularly stacked silica spheres roughly 0.2 micron across; body colours come from impurities — iron for fire opal's yellow-orange-red, clays and other pigments for pink, blue and green common opals |
| Typical inclusions | No diagnostic crystal inclusions (amorphous); look for play-of-colour patch structure, crazing cracks, matrix remnants, and columnar structure parallel to deposition |
| Stability | Stable to light; heat and sudden temperature change cause crazing (fine cracking); attacked by hydrofluoric acid and strong alkalis; hydrophane opal absorbs water temporarily |
| Heat sensitivity | High — never steam, ultrasonic or torch; even hot display lights can stress stones |
| Care and cleaning | Warm soapy water only, brief and gentle; never ultrasonic or steam; avoid dry heat, and store Ethiopian hydrophane away from dyed liquids and chemicals |
| Birthstones | October (with tourmaline); 14th wedding anniversary |
| Availability | Australia still leads supply; Ethiopian Welo opal since about 2008 transformed availability of crystal and body-coloured material; fine black opal remains scarce |
| Recognition | Excellent as an October birthstone; play-of-colour is instantly recognised by the public |
Varieties (6)
Pink-Red Opal
| Colour | pink to pinkish red bodycolour |
| Transparency | variable |
| Clarity type | n/a; usually translucent to opaque |
| Phenomena | Rare pink precious opal shows play-of-colour |
| Appearance | Soft pastel pink to rose, waxy and opaque like a harder rose quartz; Peruvian pink is the classic source |
| Trade names | Peruvian pink opal • Andean opal (with the blue opal of the same region) |
| Misnomers | sometimes marketed as rose opal or wrongly compared to kunzite; it is common opal |
| Typical sizes | Beads and cabochons of all sizes; large clean pieces available |
| Cutting styles | Beads, cabochons, carvings; rarely faceted |
| Key separations | vs rose quartz: RI 1.44-1.46 vs 1.544-1.553 and quartz is harder; vs pink chalcedony: RI and structure; vs dyed howlite or magnesite: dye in surface cracks, lower hardness; vs pink glass: bubbles and homogeneity |
| Sources (localities) | Peru (Andes) • Mexico • USA (Oregon, Nevada) • Australia |
| Availability | Steady from Peru |
| Market notes | Often stabilized or impregnated because raw material is porous; disclose |
| Care notes | Warm soapy water; porous — keep away from chemicals and prolonged soaking |
| In short | Pink opal is soft-hued common opal from Peru and Mexico, used mostly in beads and cabochons and often stabilized. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–20.00 | $5.00 – $40.00 | commercial | 2026-10-26 |
Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.
Green Opal
| Colour | green bodycolour, common or precious |
| Transparency | variable |
| Clarity type | n/a; translucent to opaque |
| Phenomena | Play-of-colour in precious green opal |
| Appearance | Apple to deep forest green; the precious kind flashes green-blue pinfire on a green ground |
| Trade names | prase opal (leek-green chalcedonic) • chrysopal (apple green) |
| Misnomers | not to be confused with chrysoprase (chalcedony) or prasiolite (green quartz) |
| Typical sizes | Cabochons and carvings of all sizes |
| Cutting styles | Cabochons, beads, carvings; precious material as cabochon or freeform |
| Key separations | vs chrysoprase: chrysoprase is chalcedony at RI 1.53-1.54 with granular structure; vs green jade: hardness and RI; vs green glass: bubbles; vs serpentine: hardness and SG |
| Sources (localities) | Peru • Brazil • Tanzania • Ethiopia • Australia • USA |
| Availability | Regular supply of common green; precious green scarcer |
| Market notes | Modest prices; often used in carvings and beads |
| Care notes | Warm soapy water; avoid heat |
| In short | Green opal spans common apple-green material and rare green precious opal with play-of-colour, mostly from Peru, Brazil and Ethiopia. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–20.00 | $5.00 – $50.00 | commercial (common opal) | 2026-10-26 |
Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.
Fire Opal
| Colour | yellow, orange to red bodycolour, with or without play-of-colour |
| Transparency | variable |
| Clarity type | n/a; transparent to opaque |
| Phenomena | Play-of-colour in some stones; the body colour itself is not 'fire' in the dispersion sense |
| Appearance | Warm glowing jelly of sunset orange, cherry red or gold; transparent stones are unusually faceted for an opal |
| Trade names | Mexican opal • cherry opal (red) • sun opal • gold opal • cantera (opal in rhyolite matrix) |
| Misnomers | 'fire' refers to body colour, not dispersion — do not sell it as a fiery (dispersive) gem |
| Typical sizes | 1 to 20 ct common; larger clean transparent stones available |
| Cutting styles | Faceted (unusual among opals) for transparent clean stones; cabochons and freeforms for play-of-colour material |
| Key separations | vs citrine: RI 1.37-1.43 vs 1.544-1.553 and lower hardness; vs amber: hot-point smell, SG; vs orange glass: bubbles; vs spessartite garnet: RI and SG far higher; hydrophane Ethiopian fire opal sticks to tongue and absorbs water |
| Sources (localities) | Mexico (Queretaro, Jalisco) • Ethiopia (Welo) • Brazil (Piaui) • Honduras • USA |
| Availability | Steady Mexican and Ethiopian supply; fine cherry red with play-of-colour scarce |
| Market notes | Mexican cantera (matrix) opals are popular; Ethiopian material is cheaper but often hydrophane |
| Care notes | Warm soapy water; very heat sensitive, crazes under dry heat; Mexican stones historically prone to crazing |
| In short | Fire opal is the warm yellow-orange-red body-coloured opal of Mexico and Ethiopia, transparent enough to facet, sometimes with play-of-colour. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–5.00 | $10.00 – $100.00 | commercial to fine | 2026-10-26 |
| 5.00–20.00 | $30.00 – $250.00 | fine (cherry red or with play-of-colour) | 2026-10-26 |
Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.
Water Opal (Colourless)
| Colour | colourless to near-colourless, transparent |
| Transparency | transparent |
| Clarity type | n/a |
| Phenomena | Play-of-colour in crystal opal grade; jelly opal has little or none; hydrophane Ethiopian stones turn clearer when soaked |
| Appearance | Glass-clear jelly with flashes of spectral colour floating inside the stone rather than on a body colour |
| Trade names | crystal opal (with play) • jelly or water opal (little or no play) • hydrophane (absorbs water) |
| Misnomers | Muller's glass refers to hyalite, the glassy colourless opal crust |
| Typical sizes | 1 to 10 ct typical for fine crystal opal |
| Cutting styles | Cabochons and freeforms on dark or clear backs; doublets common to protect thin seams |
| Key separations | vs colourless glass: bubbles, RI about 1.5 vs 1.44-1.46; vs hyalite: same species; vs synthetic opal: synthetic shows lizard-skin or columnar structure under magnification and often phosphoresces; hydrophane sticks to the tongue and wets internally |
| Sources (localities) | Ethiopia (Welo) • Mexico • Australia (Coober Pedy, Lightning Ridge) • Oregon (thundereggs) |
| Availability | Good since the Ethiopian discoveries; Australian crystal opal finer and pricier |
| Market notes | Ethiopian hydrophane crystal opal flooded the mid market; top Australian crystal black-opal-grade remains premium |
| Care notes | Warm soapy water; hydrophane stones change clarity when wet — dry slowly, never force-dry |
| In short | Water or crystal opal is transparent, body-colourless opal whose colour seems suspended inside the stone. Ethiopian Welo material dominates supply today. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–5.00 | $20.00 – $200.00 | commercial to fine | 2026-10-26 |
| 5.00–15.00 | $80.00 – $600.00 | fine Australian crystal | 2026-10-26 |
Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.
White Opal
| Colour | white to milky light body with play-of-colour |
| Transparency | variable |
| Clarity type | n/a; translucent to opaque |
| Phenomena | Play-of-colour; rare cat's-eye and asterism |
| Appearance | Pale porcelain ground flashing pinfire and harlequin patches; lively in daylight, softer in dim light |
| Trade names | white or milk opal • semi-black or gray opal sits between white and black |
| Misnomers | none widespread; beware glass and plastic imitations including Slocum stone |
| Typical sizes | 1 to 15 ct common; larger from Coober Pedy |
| Cutting styles | Cabochons, freeforms, doublets and triplets from thin seams; calibrated ovals for rings |
| Key separations | vs synthetic Gilson-type opal: lizard-skin mosaic structure, columnar pattern, phosphorescence; vs Slocum glass: large angular colour flakes, RI about 1.5; vs opal doublet: straight layer seam visible from the side; vs moonstone: RI 1.52-1.53 and adularescence rather than patchy play |
| Sources (localities) | Australia (Coober Pedy, Mintabie, Andamooka) • Ethiopia (Welo) • Brazil • Czech Republic (historic) |
| Availability | Plentiful; the volume commercial precious opal |
| Market notes | Body tone (N7 to N9) grades value; doublets and triplets give the look at a fraction of solid stone price |
| Care notes | Warm soapy water; no ultrasonic or steam; avoid dry heat |
| In short | White opal carries its play-of-colour on a pale milky body — the most available precious opal, led by Coober Pedy and Ethiopia. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–5.00 | $10.00 – $150.00 | commercial to fine | 2026-10-26 |
| 5.00–15.00 | $50.00 – $400.00 | fine | 2026-10-26 |
Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.
Black Opal
| Colour | very dark grey to black body with vivid play-of-colour |
| Transparency | variable |
| Clarity type | n/a; opaque to translucent |
| Phenomena | Play-of-colour at its most vivid; harlequin pattern most prized; rare cat's-eye |
| Appearance | Midnight ground against which spectral flashes burn with maximum contrast — the aristocrat of opals |
| Trade names | black opal (N1-N4 body tone) • black boulder opal (dark seam on ironstone) • Lightning Ridge type |
| Misnomers | smoke- or sugar-treated dark Ethiopian or matrix opal sold as black opal — treatment must be disclosed |
| Typical sizes | 1 to 10 ct typical; fine calibrated stones prized |
| Cutting styles | Solid cabochons and freeforms; dark basements left thick to preserve body tone; doublets from thin dark seams |
| Key separations | vs treated Ethiopian opal: dye or carbon concentrated in cracks and pits, colour layer skin deep; vs black boulder doublet: ironstone backing visible at seam; vs synthetic black opal: mosaic structure and phosphorescence; vs black glass: no play-of-colour structure |
| Sources (localities) | Australia (Lightning Ridge) • Ethiopia (Wegel Tena, natural dark) • Honduras (black matrix) |
| Availability | Scarce; Lightning Ridge output limited, fine stones command the highest opal prices |
| Market notes | Body tone N1-N4 plus pattern (harlequin, broadflash, flag) drive price; red multi-colour harlequin on N1 is the trophy |
| Care notes | Warm soapy water only; solid stones reasonably durable in wear, still avoid heat and knocks |
| In short | Black opal, the Lightning Ridge treasure, shows the strongest possible play-of-colour on a dark body. Body tone and pattern set its celebrated prices. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–3.00 | $100.00 – $1500.00 | commercial to fine | 2026-10-26 |
| 3.00–10.00 | $500.00 – $5000.00 | fine Lightning Ridge | 2026-10-26 |
Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.
Sources
GIA gem encyclopedia · mindat.org · Wikipedia
Data compiled and fact-checked against gemmological literature, October 2026 (JDMIS gem reference). Properties are ranges accepted in gemmological practice; individual stones vary.
See also
Gemstone reference entries: gem #18 · gem #99 · gem #122 · gem #150 · gem #175 · gem #219 · gem #229
Related materials: Corundum · Diamond · Quartz · Spinel · Topaz · Tourmaline
Identification wizard · All 83 materials · Singapore gem labs