amorphous Composite assembled stone
Opal Doublet (Cabochon)
Formula 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)
Physical Properties
- Crystal System
- amorphous
- Hardness (Mohs)
- 5.5
- Specific Gravity
- 1.9
- Lustre
- Vitreous to resinous
Optical Properties
- Refractive Index
- 1.44
- Optical Character
- Isotropic (amorphous)
- Phenomenon
- Play of colour
Origin & Identification
- Origin
- Composite
- Diagnostic Features
- Visible 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
Characteristics
Colours
Black opal appearanceVarious play of colour
Common Inclusions
Junction planeDark backing visible from sideAdhesive layer
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)