Red coral is the stone of Mars (Mangal) in the Navratna and the only one of the nine that was an animal colony rather than a mineral or a single organism's secretion. Precious coral (Corallium species, principally from the Mediterranean and the Pacific) is a calcium carbonate skeleton with a structure of fine longitudinal grain and, in cross-section, a pattern of tiny radiating channels. Genuine material of good colour is scarce and regulated in trade; the market is heavy with substitutes.
What precious coral is
Calcite (calcium carbonate) skeleton with organic matter. Properties: SG 2.60–2.70, hardness 3–4, opaque, colour from pale pink through salmon and oxblood to deep red, and — the key feature — a visible longitudinal striation (fine parallel lines along the branch axis) and radial pattern in cross-section, visible at magnification. It effervesces weakly in acid (a destructive test, not used by the laboratory) and takes a good polish.
What is sold as red coral
Dyed white or pale coral. Genuine coral of low colour, dyed red. The most common deception. Dye concentrates in surface pits, cracks and the drill hole; a swab moistened with acetone may lift colour (the laboratory checks in an inconspicuous spot or uses UV and magnification instead); colour under magnification sits in the structure rather than being the structure.
Reconstituted (pressed) coral. Coral powder bound with resin, dyed, and pressed into shape. Uniform colour, no longitudinal grain, sometimes a granular texture under magnification, resin fluorescence under UV, SG lower than solid coral.
Bamboo coral, dyed. A different, inexpensive coral with a pronounced segmented structure, dyed red and sold as precious coral. The segment joints and coarser structure are visible.
Dyed bone and dyed shell. Bone shows Haversian canals (a fine dotted or channelled structure) rather than coral's striation; shell shows a layered, curved structure. Both take dye unevenly.
Plastic and resin. No grain, gas bubbles, warm to touch, low SG, mould marks, fluorescence.
Glass. No grain, bubbles, conchoidal chips, SG variable.
Gilson and other manufactured "coral". Calcite-based synthetic material with no natural grain.
How the laboratory examines coral
Magnification for longitudinal striation, radial cross-section pattern, and the distribution of colour. Weight and SG. UV fluorescence (natural coral is usually inert or dull; resin binders and some dyes fluoresce). Examination of the drill hole and any surface pits for dye concentration. Spectroscopy where available for dye. Report states: natural precious coral or otherwise, and whether dyed, reconstituted or imitation.
Treatments in genuine coral
Dyeing, as above — the principal treatment. Bleaching to lighten. Impregnation with wax or resin to fill pores and improve polish; detected by UV and at the surface. Coating. All must be disclosed.
Colour and grading
Colour terms in the trade — oxblood, salmon, angel skin — describe shade and are matters of preference. Deep, even red without white patches is most prized. Grading is opinion; identity is the finding.
Legal note
Several Corallium species are listed under wildlife-trade regulations in various jurisdictions, and import or export of precious coral may be controlled. A laboratory report identifies the material; it does not address legality, which the buyer should check for their own location.
Frequently asked questions
How can I tell dyed coral at home?
Look at the drill hole and any pits under a loupe for concentrated dye; uneven, unnatural colour is a warning. A laboratory examination is definitive.
Is reconstituted coral genuine coral?
It contains coral powder. It has none of the structure of natural coral and should be sold as reconstituted, not as coral.
Does coral have a hardness test?
It is soft (3–4) and easily scratched; the laboratory does not scratch it. Hardness is inferred from identity.
Why is red coral expensive?
Precious coral grows slowly in deep water, harvesting is restricted, and deep even red is rare. The price attracts every kind of substitute.
Is coral suitable for daily wear?
It is soft and porous; rings suffer. Keep from perfumes and acids and wipe after wear. Organic gems are soft.