Coral (Indian: Moonga; Italian: Corallo) is an organic substance formed as a branch like calcareous framework by a microscopic marine organism known as coral polyp. It is formed by the accretion of colonies of these tiny marine animals. The two main species are the corallium rubrum and corallium nobile.

Bamboo corals (Gorgonian Coral) from Taiwan are less-expensive, highly porous and gray to white color which are dyed in several colors, mostly in red, and sold as red coral. Absence of closely packed parallel line on the surface of sea bamboo coral is the key identifying feature to separate it from red corals. Generally, the gap between two parallel lines on the surface of sea bamboo coral is much more than that found on natural red coral.

Natural corals are sourced off the coasts of Hawaii which is then exported to Italy (called Italian coral) and then re-exported through out the world. Natural coral with combination of red and orange color (red color being primary while orange color being secondary) has the best value.

Golden coral is precious due to its rarity; pink and red coral is used as bead in jewelry.

Classification / Type: 

1. Calcareous Coral: Carbonate type (white, pink, red and orange) display a distinctive pattern of parallel stripes of slightly different color and transparency.
2. Conchiolin Coral: Horny type (black, blue, golden and brown) display a concentric structure about the axis of the original branch. Generally, golden coral displays a distinctive fine pimply appearance, in addition to a concentric structure and is some cases a faint sheen.

Colors / Varieties: 

White, pink, red, orange, blue to violet, golden, brown and black.

Transparency: 

Semi-translucent to opaque.

Crystal System / Forms: 

None. Occurs in a branch like form.

Hardness: 

3.5

Specific Gravity: 

• Calcareous Coral: 2.60 - 2.70
• Conchiolin Coral: 1.30 - 2.20

Cleavage / Fracture: 

None / Conchoidal to Uneven.

Optic Character: 

Generally A.G.G. or opaque.

Lustre: 

Waxy to vitreous.

Refractive Index / Birefringence: 

• Calcareous Coral: 1.486 - 1658 / 0.172
• Conchiolin Coral: 1.56

Pleochroism: 

None.

Dispersion: 

None.

Magnification: 

Wavy, parallel (whitish line along the length of the stone), fibrous structure, tiny cavities (polyp holes); conchiolin coral (horny type coral) may exhibit tree-ring structure.

U.V. Fluorescence: 

Mostly inert or dull purplish red.

Spectrum: 

Not diagnostic.

Treatment (Enhancement): 

• Colored impregnation (dyeing): light colors to deep colors.
• Bleaching (all colors)
• Coating
• Colorless impregnation
• Molding

Specific Tests: 

• Calcareous Coral: Reacts with vigorous effervescence to dilute acids, such as hydrochloric acid (HCl).
• Conchiolin Coral: Does not effervesce with acids. Black coral may be carefully tested with a hot point for its characteristic burnt hair odor.

Synthesis: 

Gilson created coral (reconstructed coral): No parallel lines (veins) but dark angular spots or patches are seen.

Simulants (with separation tests): 

Glass (structure, S.G.); Plastic (structure, hot point); Shell (structure); Stained marble (structure); Conch pearl (structure); Wood (structure); Horn (structure), etc.

Geological Occurrence: 

Sea bed.

Sources: 

Grows in warm seas (13°C - 16°C), found off the coasts of Hawaii, from the Bay of Biscay through into the Mediterranean sea, Red sea, Japanese coast. Black coral is found off the Cameroon coast.

Cuts & Uses: 

Used for beads, carving and cabochons, rough coral branches sometimes strung into necklaces, carved into ornaments.