Gem library
Coral
The red skeleton of the Mediterranean
Jewellery coral is not a plant but the calcareous skeleton of a colony of animals, built branch after branch over decades. It is also the gem whose exploitation raises the most direct questions, and they are better looked at squarely.
- Hardness (Mohs)
- 3 – 4
- Density
- 2.60 – 2.70
- Crystal system
- Trigonal
- Refractive index
- 1.486 – 1.658
- Cleavage
- None
Mineralogical identity
- Family
- Organic gem (biomineral)
- Species
- Corallium rubrum and related species
- Variety
- Red coral, angel skin, black coral
- Chemical formula
- CaCO₃ (calcite) + organic matter
- Crystal system
- Trigonal (calcite in fibres)
- Classification
- Organic gem
- Rarity
- Uncommon
- Indicative price
- €5 – 300/g (angel skin: far beyond)
Etymology
From the Greek korallion, by way of the Latin corallium. The origin of the Greek word is still debated, perhaps from a Semitic root meaning a small pebble.
Physical & optical properties
- Hardness (Mohs)
- 3 – 4
- Density
- 2.60 – 2.70
- Refractive index
- 1.486 – 1.658
- Birefringence
- 0.160
- Cleavage
- None
- Fracture
- Uneven
- Lustre
- Waxy to vitreous when polished
- Diaphaneity
- Opaque
- UV fluorescence
- None
- Pleochroism
- None
- Colours
- red, orange red, pink, white, black
Formation, geology & history
An animal, not a plant
The red coral of the Mediterranean, Corallium rubrum, is a colonial cnidarian: thousands of polyps, relatives of jellyfish and anemones, live on the surface of a calcareous axis they secrete. That axis, made of fibrous calcite pigmented by carotenoids, is the part that is worked. Unlike the reef-building corals, Corallium rubrum does not live in symbiosis with algae and so has no need of light: it is found between thirty and eight hundred metres down, on rock walls and in submarine caves. Its growth is slow, of the order of a millimetre of diameter a year: a branch the size of a finger is often half a century old.
The varieties and their names
The market distinguishes several qualities. Mediterranean blood red, called sciacca or Torre del Greco coral depending on origin, remains the historic reference. Angel skin coral, a very pale and uniform pink, fished mainly off Japan and Taiwan, is the dearest by weight. White coral and the orange kind called momo come from the same deep Pacific waters. Black coral, on the other hand, is not calcareous at all: its skeleton is made of conchiolin, a protein, which makes it an entirely different material despite the common name. Torre del Greco, near Naples, has concentrated most of the world's coral working since the eighteenth century.
What to know before buying
Red coral populations have fallen sharply: overfishing, slow growth, warming and episodes of mass mortality. Several species of the genus Corallium are listed in CITES Appendix III, and fishing is capped in the Mediterranean, with minimum sizes and quotas. Buying old coral, taken from a family jewel or a second-hand market, removes nothing from the sea. Buying new requires asking about origin and regulatory status, and a serious dealer knows both. Finally, the vast majority of cheap coral sold today is not coral: it is magnesite or howlite dyed red.
How to recognise it
- Longitudinal striaeUnder a loupe, coral shows striae parallel to the axis of the branch and, in cross-section, concentric growth rings. A dyed paste has neither.
- The acid testA drop of vinegar or lemon juice on a hidden part fizzes on a carbonate. Black coral, being a protein, does not react. A test to use only as a last resort.
- Dye in the hollowsRub a discreet corner with cotton soaked in acetone: a dyed stone bleeds. Natural coral, pigmented throughout, leaves nothing.
Imitations, treatments & confusions
- Dyed howlite and magnesiteBy far the most widespread imitation. White to begin with, veined with grey, they take red very well. The dye shows in the veins, which are darker.
- Reconstituted coralCoral powder bound with resin, sometimes sold without saying so. A homogeneous mass, no growth striae, and a lower density.
- Plastic and dyed sea bambooSea bamboo is a real coral, but porous and whitish, systematically dyed and impregnated. It is worth a fraction of the price of Mediterranean red.
Deposits & origins
Italy (Sardinia, Sicily, the red coral of the Mediterranean), Spain, Algeria, Tunisia, Japan and Taiwan (deep-sea coral), Hawaii
Care & precautions
- Never prolonged water or acid products: coral is a carbonate, and vinegar and descalers dissolve it.
- No perfume or hairspray, which dull it within a few applications.
- Wipe with a soft cloth after wearing, perspiration alone is enough to matt the polish.
- Store away from light: the red fades slowly and irreversibly.
Traditions & crystal healing
- Chakras
- Root · sacral
- Zodiac signs
- Aries · Scorpio
- Element
- Water
Worn as an amulet since antiquity, notably as a raw branch against the evil eye throughout the Mediterranean basin, and tied to the protection of children. It is also given for the thirty-fifth wedding anniversary. Cultural heritage, never medicine.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Is buying coral an ecological problem?
For new coral, yes, the question arises: very slow growth, falling populations, capped fishing. Old or second-hand coral removes nothing, and it is the simplest choice.
Is my red coral likely to be fake?
Statistically, yes, if it is cheap. Dyed howlite and magnesite dominate the entry-level market. Look for growth striae under a loupe.
Is black coral coral?
It is a coral in the biological sense, but not in the material sense: its skeleton is made of protein, not limestone. It does not fizz in acid and is worked like horn.
Tools for this stone
Coral: only the tools this stone actually concerns, each carrying the value read on this page.