Coral

Precious coral species including red (ox blood), pink (angel skin), and related varieties with identification, treatments, and conservation.

By Fabian Moor Last updated
species/coral precious-coral ox-blood angel-skin organic

Introduction

Precious coral is the calcified skeleton of deep-water colonial marine organisms of
genus Corallium, composed of calcium carbonate (calcite form) with carotenoid pigments.
It has no crystal system — a fibrous calcite aggregate — classifying it as an organic
gem. Diagnostic properties: hardness 3–4 Mohs, SG 2.60–2.70, spot RI 1.486–1.658,
waxy to vitreous lustre, and a concentric wood-grain growth pattern under magnification.
[1] Effervescence with hydrochloric acid confirms the carbonate composition and
separates coral from dyed howlite or plastic. Colour is the primary value driver: deep
saturated "ox blood" red from Corallium japonicum off Japan and Taiwan commands the
highest prices, followed by pale-pink "angel skin" from Corallium elatius, then common
salmon-orange Mediterranean material (Corallium rubrum). Precious coral grows as little
as 1 mm per year at depths of 100–300 metres, making it a finite resource subject to
CITES trade controls and requiring documentation for international trade.

Biology and Formation

Understanding precious coral:

What Is Coral?

  • Colonial marine organisms (polyps)
  • Secrete calcium carbonate skeleton
  • Deep water (100–300+ metres) [1]
  • 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 [1]

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

Care and Durability

References

  1. ↑ 1. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.

Related Topics

Gemstone Treatments