Gemstone profile and identification guide
Opal is less about crystal form than about light. Depending on structure, it may show a soft inner haze, bold flashes of spectral colour, or a calm body tone that changes character as the stone is turned in the hand.
Opal is less about crystal form than about light. Depending on structure, it may show a soft inner haze, bold flashes of spectral colour, or a calm body tone that changes character as the stone is turned in the hand.
Did you know? Unlike most gemstones, opal has no regular crystal lattice; its optical effects depend on internal structure rather than a standard crystal form.
Use this profile as a collector reference and educational guide for quick on-page comparison, then validate with the interactive identification system.
Name: Opal
Scientific classification: Mineraloid (Opal)
Hardness: 5.5 Mohs
Transparency: transparent to opaque
Lustre: vitreous to waxy
Crystal system: Amorphous
Primary colour: Multicolour
Secondary colour: Not specified
Colour family: Special Optical Effects
Check whether the colour play, if present, moves with the viewing angle and appears internal rather than painted onto the surface. Its lower hardness and amorphous structure also set it apart from quartz lookalikes.
Status: verified-core-fields
Reference priority: GIA, Gem-A, Mindat, International Gem Society
Core identity fields reviewed against consensus gemological references (GIA/Gem-A/Mindat/IGS).
Some market- and locality-level details may still require citation-grade source notes for publication workflows.
Hardness category: soft
Hardness value: 5.5 Mohs
Chemical composition: Hydrated silica (SiO2.nH2O)
Crystal system: Amorphous
Common locations: Australia (notably Coober Pedy, Lightning Ridge), Ethiopia (Wollo), Mexico, Brazil
Learn more: Gem-A gemstone education resources.
Opal is listed in the special optical effects grouping, with multicolour as the dominant body colour. Some specimens also show opalescence, play of colour, iridescence, which can alter the way the colour is perceived under changing light.
Optical effects: Play of Colour, Opalescence
Mineral species: Opal
Mineral group: Opal mineraloid
Chemical formula: Hydrated silica (SiO2.nH2O)
Mohs hardness: 5.5
Crystal system: Amorphous
Lustre: vitreous to waxy
Transparency: transparent to opaque
Streak: White
Cleavage: None
Fracture: Conchoidal to uneven
Specific gravity: 1.98-2.25
Refractive index: 1.37-1.52
Optical phenomena: Play of Colour, Opalescence
Colour range: Multicolour
Common inclusions: Potch layers, crazing in unstable material, and growth banding
Geological environment: Opal occurs in Low-temperature silica deposition in sedimentary or volcanic host rocks. Its Amorphous structure reflects the pressure-temperature conditions of those host rocks, with notable occurrences in Australia (notably Coober Pedy, Lightning Ridge), Ethiopia (Wollo), Mexico, Brazil.
Principal mining regions: Australia (notably Coober Pedy, Lightning Ridge), Ethiopia (Wollo), Mexico, Brazil
Important commercial sources: Australia remains the historic dominant source; Ethiopia is a major modern source
Historical significance: Opal has been prized for play-of-color since classical antiquity.
Common treatments: Stabilization, smoke/sugar treatments, and assembled doublet/triplet products occur in trade.
Synthetic or imitation forms: Synthetic opal and opalescent glass imitations are widely available.
Common look-alikes: peruvian blue opal, boulder opal, crystal opal
Jewellery uses: Opal is used chiefly for Often best in pendants, earrings, collector cuts, and occasional-wear rings that showcase colour without unnecessary risk.. In the trade it is commonly fashioned as faceted stones, calibrated cuts, and higher-clarity collector pieces, with Mohs hardness 5.5 guiding suitable jewellery applications.
Care recommendations: Opal should be cleaned with lukewarm water, mild soap, and a soft brush. At Mohs 5.5 in the Amorphous system, avoid steam cleaners, harsh chemicals, and high-impact wear. Store separately from harder gems to minimise abrasion and edge wear.
Cross-check profile data with GIA Gem Encyclopedia and Mindat entry for Opal.
Opal is a Opal material with chemistry Hydrated silica (SiO2.nH2O). It develops in Low-temperature silica deposition in sedimentary or volcanic host rocks, and economically significant production is associated with Australia (notably Coober Pedy, Lightning Ridge), Ethiopia (Wollo), Mexico, Brazil.
Australia remains the historic dominant source; Ethiopia is a major modern source
Best for: Opal is used chiefly for Often best in pendants, earrings, collector cuts, and occasional-wear rings that showcase colour without unnecessary risk.. In the trade it is commonly fashioned as faceted stones, calibrated cuts, and higher-clarity collector pieces, with Mohs hardness 5.5 guiding suitable jewellery applications.
Ideal uses: Often best in pendants, earrings, collector cuts, and occasional-wear rings that showcase colour without unnecessary risk.
Opal should be cleaned with lukewarm water, mild soap, and a soft brush. At Mohs 5.5 in the Amorphous system, avoid steam cleaners, harsh chemicals, and high-impact wear. Store separately from harder gems to minimise abrasion and edge wear.
Need another candidate? Use advanced gemstone search or return to the identification guide.
Compare hardness (5.5 Mohs), crystal system (Amorphous), colour profile, and lustre (vitreous to waxy) with your sample. Then cross-check related gemstones listed above.
Opal is listed in the special optical effects grouping, with multicolour as the dominant body colour. Some specimens also show opalescence, play of colour, iridescence, which can alter the way the colour is perceived under changing light.
Often best in pendants, earrings, collector cuts, and occasional-wear rings that showcase colour without unnecessary risk.. For maintenance, follow the care guidance in this profile.
GIA: general gemstone identity (reviewed) - Reviewed as a direct core gemstone profile.
Gem-A: gemological education and terminology (reviewed) - Reviewed as a direct core gemstone profile.
Mindat: mineral species and locality index (reviewed) - Reviewed as a direct core gemstone profile.
International Gem Society: collector-facing gem reference (reviewed) - Reviewed as a direct core gemstone profile.
