amorphous Tectosilicates
Chalcedony (Botryoidal)
Formula SiO2
Crystal Structure
#! Species: Chalcedony
#! System: Trigonal (32)
#! Habit: Botryoidal grape-like form
amorphous[cryptocrystalline]:{botryoidal}
amorphous
32
{botryoidal}
Quick Facts
Hardness
6.5
Specific Gravity
2.58
Refractive Index
1.53
Optical Character
Aggregate
Physical Properties
- Crystal System
- amorphous
- Hardness (Mohs)
- 6.5
- Specific Gravity
- 2.58
- Cleavage
- None
- Fracture
- Conchoidal
- Lustre
- Waxy to vitreous
Optical Properties
- Refractive Index
- 1.53
- Birefringence
- 0.004
- Optical Character
- Aggregate
Characteristics
Colours
Blue (chalcedony)Red-orange (carnelian)Green (chrysoprase)Banded (agate)Black (onyx)Brown (sard)
Localities
BrazilUruguayIndiaMadagascarUSAGermany (Idar-Oberstein)
Common Inclusions
Banding patternsMoss-like dendritic inclusions (moss agate)Plume inclusions (plume agate)
Known Treatments
DyeingHeat treatmentSugar-acid treatment (onyx)
Crystal Forms
botryoidal
Diagnostic Features
Cryptocrystalline; waxy lustre; banded (agate) or uniform colour; lower RI than crystalline quartz
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 (Massive)
SiO2
Chrysocolla
(Cu,Al)2H2Si2O5(OH)4.nH2O
Copal (Amber Simulant)
Partially polymerised plant resin (labdanoid diterpenes; younger resin; not fully matured/polymerised)
Coral
CaCO3 (calcareous; polycrystalline calcite + organic matrix); black coral = gorgonin protein