amorphous Composite assembled stone
Opal Triplet (Cabochon)
Formula SiO2*nH2O between dome cap and dark backing
Crystal Structure
amorphous
Quick Facts
Hardness
5.5
Specific Gravity
1.9
Refractive Index
1.44
Optical Character
Isotropic
Physical Properties
- Crystal System
- amorphous
- Hardness (Mohs)
- 5.5
- Specific Gravity
- 1.9
- Lustre
- Vitreous (glass-like from dome cap)
Optical Properties
- Refractive Index
- 1.44
- Optical Character
- Isotropic
- Phenomenon
- Play of colour
Origin & Identification
- Origin
- Composite
- Diagnostic Features
- Three-layer construction visible from side, dome cap (quartz or glass) creates glassy appearance, flat back with dark backing, play of colour may appear slightly muted under dome, cannot be immersed (adhesive dissolves)
Characteristics
Colours
Black opal appearanceVarious play of colour
Common Inclusions
Three-layer constructionDome capJunction planesDark backing
Natural 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)