amorphous Organic gems
Coral
Formula CaCO3 (calcareous; polycrystalline calcite + organic matrix); black coral = gorgonin protein
Crystal Structure
#! Species: Coral (Corallium rubrum)
#! System: Amorphous (biomineral aggregate)
#! Habit: Arborescent colonial growth; polycrystalline calcareous skeleton
amorphous[waxy]:{massive} | phenomenon[organic:coral]
amorphous
none
{massive}
Quick Facts
Hardness
3.0
Specific Gravity
2.6
Refractive Index
1.486
Optical Character
AGG
Physical Properties
- Crystal System
- amorphous
- Hardness (Mohs)
- 3.0
- Specific Gravity
- 2.6
- Cleavage
- None (polycrystalline aggregate)
- Fracture
- Uneven to splintery
- Lustre
- Waxy to sub-vitreous
Optical Properties
- Refractive Index
- 1.486
- Optical Character
- AGG
- Pleochroism
- None
Characteristics
Colours
Red (oxblood/rosso grade)Orange-redPale pink (angel skin)WhiteBlack (black coral)
Localities
Mediterranean Sea (Tunisia, Algeria, Morocco, Italy - Torre del Greco)Japan and Taiwan (Corallium japonicum and related species)Hawaii (Pleurocorallium secundum)Midway Atoll (deep-water gold coral)
Common Inclusions
Growth tubes and structural voidsOrganic matrix bands
Known Treatments
Dyeing and colour enhancementBleaching and re-dyeingResin impregnation
Crystal Forms
massivearborescent
Diagnostic Features
Longitudinal growth striations (most diagnostic); effervescence in HCl (calcareous coral); SG 2.60-2.70; low RI on spot reading; wood-like organic microstructure under magnification
Notes
Not an IMA mineral species; biomineral product of Corallium rubrum (Mediterranean) and related colonial organisms. Calcareous skeleton composed of polycrystalline calcite + organic scleroprotein matrix. CITES Appendix II (regulated trade). SG and RI per Read 7th ed. (DOI 10.4324/9780080507224); species identification methods confirmed by Lendvay et al. 2020 (DOI 10.1038/s41598-020-64582-4) and Vielzeuf et al. 2021 (DOI 10.15506/jog.2021.37.6.596). IMA-approved: N/A (biological organism).
Synthetics & Simulants
Known synthetic and simulant versions of Coral
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)