Coral
Precious coral species including red (ox blood), pink (angel skin), and related varieties with identification, treatments, and conservation.
species/coral precious-coral ox-blood angel-skin organic
Introduction
Precious coral is the calcified skeleton of colonial marine animals (genus
Corallium). Prized for millennia, particularly in the Mediterranean cultures,
fine red coral remains valuable despite conservation concerns.
Unlike reef-building corals (which are not used in jewellery), precious
coral grows in deep water and has a solid, polishable structure.
Biology and Formation
Understanding precious coral:
What Is Coral?
- Colonial marine organisms (polyps)
- Secrete calcium carbonate skeleton
- Deep water (100-300+ metres)
- Slow growing (1mm per year or less)
- Tree-like branching structure
Composition
- Material: Calcium carbonate (calcite form)
- Colour agents: Carotenoid pigments
- Structure: Concentric growth layers
- Grain: Wood-like growth pattern visible when polished
Physical Properties
| Property | Value |
|---|---|
| Hardness | 3-4 Mohs (soft) |
| Specific gravity | 2.60-2.70 |
| Refractive index | 1.486-1.658 (spot) |
| Character | Aggregate; often shows grain |
| Lustre | Waxy to vitreous when polished |
| Composition | Calcium carbonate (calcite) |
Precious Coral Types
| Species | Common Name | Colour | Source |
|---|---|---|---|
| Corallium rubrum | Mediterranean coral | Red to pink | Mediterranean Sea |
| Corallium japonicum | Japanese coral | Deep red (ox blood) | Japan, Taiwan |
| Corallium elatius | Angel skin coral | Pale pink | Japan |
| Corallium secundum | Midway coral | Pale pink | Hawaii, Midway |
| Corallium konojoi | White coral | White | Japan |
Colour Hierarchy
Coral value by colour:
Ox Blood Red
The most valuable coral colour:
- Deep, saturated red
- Even colour distribution
- Minimal white core
- Japanese (Corallium japonicum) origin
- Commands highest prices
Angel Skin Pink
Delicate pale pink:
- Subtle, feminine colour
- Even, translucent quality
- Popular in Victorian era
- Japanese origin prized
- Second highest value
Other Colours
- Salmon/orange: Common Mediterranean
- White: Used in jewellery; lower value
- Mottled: Uneven colour; lower value
Coral Identification
Distinguishing genuine from imitation coral:
Natural Features
- Grain pattern: Wood-like growth lines
- Colour variation: Subtle natural variation
- White core: May show in cross-section
- Weight: Heavier than most plastic
Tests
| Test | Coral | Imitation |
|---|---|---|
| Acid test (HCl) | Effervesces | No reaction (usually) |
| Hot needle | Calcite smell | Plastic/acrid smell |
| Magnification | Growth structure | Mold marks, bubbles |
| Weight | Substantial | Often lighter |
Treatments
Common coral treatments:
Dyeing
- Pale coral dyed to improve colour
- Detection: Colour in crevices; acetone test
- Very common in commercial material
- Should be disclosed
Other Treatments
- Bleaching: Lightens orange to pink
- Waxing/oiling: Improves lustre
- Impregnation: Stabilises porous material
- Coating: Surface enhancement
CITES and Conservation
Imitations and Alternatives
Common coral substitutes:
Imitations
| Material | Distinction |
|---|---|
| Dyed howlite | Different structure; lighter; no effervescence |
| Plastic | Warmer feel; lighter; no grain |
| Glass (paste) | Heavier; may show bubbles |
| Pressed coral | Reconstituted; grainy appearance |
Ethical Alternatives
- Bamboo coral: Different species; often dyed
- Fossil coral: Agatised ancient coral
- Synthetic coral: Man-made alternatives
- Sponge coral: Different structure