amorphous Mineraloid
Opal
SiO2.nH2O
Crystal Structure
#! Species: Opal
#! System: Amorphous (mineraloid)
#! Habit: Botryoidal masses with play of color
amorphous[opalescent]:{botryoidal} | phenomenon[play_of_color:intense]
amorphous
none
{botryoidal}
Quick Facts
Hardness
5.5
Specific Gravity
1.98
Refractive Index
1.37
Optical Character
Isotropic
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Open in PlaygroundPhysical Properties
Crystal Systemamorphous
Hardness (Mohs)5.5
Specific Gravity1.98
CleavageNone
FractureConchoidal
LustreVitreous to resinous
Optical Properties
Refractive Index1.37
Optical CharacterIsotropic
PleochroismNone (amorphous)
PhenomenonPlay of color (precious opal)
Colours
White body (white opal)Black body (black opal - most valuable)Transparent (crystal opal)Orange-red body (fire opal)Boulder matrixPlay of color (spectral flashes)
Localities
Australia (Lightning Ridge - black; Coober Pedy - white)Ethiopia (Welo - hydrophane)Mexico (fire opal)BrazilUSA (Nevada, Oregon)Honduras
Known Treatments
Smoke treatmentSugar/acid treatmentPolymer impregnationDoublets and triplets
Crystal Forms
botryoidalnodulesveinsreplacement forms
Diagnostic Features
Play of colour (precious); amorphous; low hardness (5.5-6.5); hydrated silica
Notes
Mineraloid — no true crystal structure; play of color from diffraction in ordered silica sphere arrays
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Open in PlaygroundSynthetics & Simulants
Known synthetic and simulant versions of Opal
Related Minerals
Other minerals in the amorphous system
Ambroid (Pressed Amber)
Compressed and fused amber chips (same chemical composition as amber — fossil resin; succinite for Baltic amber)
Bone (Ivory Simulant)
Ca5(PO4)3(OH) (hydroxyapatite; same general composition as ivory dentine) in collagen matrix; technically microcrystalline hydroxyapatite but gemologically treated as amorphous/aggregate biomineral
Chalcedony (Botryoidal)
SiO2
Chalcedony (Massive)
SiO2
Chrysocolla
(Cu,Al)2H2Si2O5(OH)4.nH2O
Copal (Amber Simulant)
Partially polymerised plant resin (labdanoid diterpenes; younger resin; not fully matured/polymerised)