What is coral? The gem that was once alive

Coral biology and gem material formation

Precious coral organisms are colonial cnidarians of the order Antipatharia and related groups, with Corallium being the primary commercial genus. The colonies grow as branching tree-like structures, the living polyps forming a thin outer layer on the calcified skeletal core that is the gem material. Growth rates are extremely slow: Corallium rubrum grows approximately 1-2 centimetres per year in diameter, meaning a branch 2 centimetres in diameter represents decades of growth. Commercial-size branches suitable for gem carving represent centuries of growth (GIA Gem Reference Guide, 2006, pp. 8-9; IUCN Red List assessments for Corallium species).

The gem material is the inner calcified skeleton, not the living tissue. The material is composed primarily of calcite (a form of calcium carbonate) with an organic protein matrix. When polished, precious coral shows a characteristic waxy to vitreous lustre that distinguishes it from other red materials (GIA; Wise, 2016).

Mediterranean sources: Sardinia and the Italian tradition

The Mediterranean, particularly the waters around Sardinia, Corsica, the Strait of Messina, the Catalan coast, and the waters off Tunisia and Morocco, historically produced Corallium rubrum, the deep red to pink Mediterranean precious coral that is the reference standard for the species. The vibrant red of the finest Sardinian Corallium rubrum, called "ox-blood" or "rosso scuro," is the colour benchmark against which all other precious coral varieties are measured (GIA; Wise, 2016, pp. 357-360).

Mediterranean Corallium rubrum populations have been severely depleted by centuries of harvesting, accelerating dramatically in the 20th century with mechanised harvesting. Current Mediterranean production is a fraction of historical levels. Italian regulations and European Union fisheries rules restrict harvest, but enforcement has been inconsistent and illegal harvesting continues.

Pacific sources: Japan and Taiwan

Japanese and Taiwanese coral harvesting in Pacific waters, particularly around the Ogasawara Islands, Midway, and the deep waters of the Pacific and Indian Oceans, produces several Corallium species including Corallium japonicum (deep red to pink) and Corallium secundum (angel skin pink to pale pink). Pacific coral has a different texture from Mediterranean Corallium rubrum: the Pacific species typically shows a more granular internal texture and a slightly different lustre character. Japanese coral carving traditions, particularly in Kochi Prefecture, are significant parallel traditions to the Italian work (GIA; Wise, 2016, pp. 360-363).

The "angel skin" variety, a delicate pale pink to near-white colour, comes primarily from Japanese harvesting of Corallium secundum and related species and has a specific collector market in Asia.

Colour range and quality

Precious coral colour ranges from deep ox-blood red through vivid red, orange-red, pink, salmon, pale pink (angel skin), and white. The colour hierarchy in commercial terms: the finest Mediterranean ox-blood red commands the highest prices, followed by vivid red, then salmon and pink, then angel skin, then white. The colour is intrinsic to the organism's biology and not produced by any external colourant. Fine precious coral should show even, consistent colour throughout the branch without pale patches, dark streaks, or white spots (GIA; Wise, 2016).

Quality factors: colour saturation and evenness, absence of pits or voids in the material (common in lower grades), polish quality (high polish reveals the colour depth; poor polish looks chalky), and the absence of cracks or repairs. Larger branches and carvings command premiums over equivalent beads of equivalent colour because the quantity of material required for a large carving from a single branch is significantly rarer than bead-grade small pieces (GIA; Wise, 2016).

CITES restrictions and conservation status

All Corallium species are listed on CITES Appendix II, which requires documentation of legal origin and sustainable harvest for any international trade. This means that any precious coral crossing an international border must be accompanied by CITES documentation confirming it was legally harvested. The practical consequence for buyers: significant precious coral jewellery purchases should include provenance documentation confirming legal origin. Without documentation, the buyer has no assurance the material was not illegally harvested (CITES; IUCN; Wise, 2016).

The US Fish and Wildlife Service has additional restrictions on coral import. Buyers in the USA should confirm that any significant precious coral purchase is accompanied by appropriate CITES and import documentation from the seller.

Treatments in precious coral

Precious coral is commonly dyed to improve or standardise colour. Pink coral is sometimes dyed to appear a more vivid red. Pale or white coral is sometimes dyed to simulate more valuable pink or red material. The dye is not stable: it fades with UV exposure and can be detected by wiping with acetone. GIA and major laboratories test for dyeing. Natural-colour precious coral commands significant premiums over dyed equivalents (GIA; AGTA treatment codes; Wise, 2016).

Filling of pits and voids with wax or resin is also practiced to improve surface appearance. This filling may obscure the natural texture and quality of the material. Examination under magnification reveals filled areas by their different lustre character.

Coral imitations

The coral imitation market is significant because natural precious coral is expensive and its colour is commercially desirable. Common imitations: dyed howlite (similar to its use as turquoise simulant), dyed bone (animal bone with similar density to coral), dyed plastic (warm to the touch unlike coral which is cool), and sponge coral (the skeletal material of different marine organisms, often dyed to simulate precious coral). A basic test: genuine precious coral shows a granular structure under magnification; plastic shows a uniform structure; bone shows a different granular pattern. Specific gravity measurement and FTIR spectroscopy are definitive tests (GIA; Wise, 2016).