amorphous Composite assembled stone
Opal Doublet (Cabochon)
SiO2*nH2O on dark backing
Crystal Structure
amorphous
Quick Facts
Hardness
5.5
Specific Gravity
1.9
Refractive Index
1.44
Optical Character
Isotropic (amorphous)
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Open in PlaygroundPhysical Properties
Crystal Systemamorphous
Hardness (Mohs)5.5
Specific Gravity1.9
LustreVitreous to resinous
Optical Properties
Refractive Index1.44
Optical CharacterIsotropic (amorphous)
PhenomenonPlay of colour
Origin & Identification
OriginComposite
Diagnostic FeaturesVisible dark backing when viewed from side, flat back, slightly different appearance from solid opal, adhesive layer may be visible at junction, profile view shows two-layer construction
Colours
Black opal appearanceVarious play of colour
Common Inclusions
Junction planeDark backing visible from sideAdhesive layer
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Open in PlaygroundNatural Counterpart
The natural gem this composite imitates
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)