amorphous Mineraloid
Opal
Formula 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
Physical Properties
- Crystal System
- amorphous
- Hardness (Mohs)
- 5.5
- Specific Gravity
- 1.98
- Cleavage
- None
- Fracture
- Conchoidal
- Lustre
- Vitreous to resinous
Optical Properties
- Refractive Index
- 1.37
- Optical Character
- Isotropic
- Pleochroism
- None (amorphous)
- Phenomenon
- Play of color (precious opal)
Characteristics
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
Synthetics & 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)