amorphous Organic gems

Coral

CaCO3 (calcareous; polycrystalline calcite + organic matrix); black coral = gorgonin protein

Crystal Structure

CDL Notation

 #! Species: Coral (Corallium rubrum)
#! System: Amorphous (biomineral aggregate)
#! Habit: Arborescent colonial growth; polycrystalline calcareous skeleton
amorphous[waxy]:{massive} | phenomenon[organic:coral]

Crystal System amorphous

Point Group none

Crystal Forms (1)

{massive}

Quick Facts

Hardness

3.0

Specific Gravity

2.6

Refractive Index

1.486

Optical Character

AGG

Physical Properties

Crystal Systemamorphous

Hardness (Mohs)3.0

Specific Gravity2.6

CleavageNone (polycrystalline aggregate)

FractureUneven to splintery

LustreWaxy to sub-vitreous

Optical Properties

Refractive Index1.486

Optical CharacterAGG

PleochroismNone

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).