Precious coral

Precious coral is the axial skeleton of a colonial marine animal — not the reef-building corals of shallow tropical water, which are a different order and make a different mineral, but the deep, cold, dark-water gorgonians of the genus Corallium.

The distinction matters mineralogically as well as commercially. Reef corals build their skeletons from aragonite. Precious coral builds from high-magnesium calcite, which is denser, harder and takes the fine polish the jewellery trade wants. The two materials are both calcium carbonate and are not interchangeable.

The colour is organic. Red and pink coral get theirs from carotenoid pigments bound into the carbonate as it is laid down, not from iron or any metal ion, which is why heat destroys the colour rather than changing it and why bleaching removes it entirely.

The animal is a colony of polyps living on the surface of a branching skeleton it secretes beneath itself. It has no symbiotic algae, so unlike a reef coral it does not need light, and it grows at depths from around thirty metres to well over a thousand — the Mediterranean beds are typically 50 to 200 metres down, on rock overhangs and cave ceilings.

Growth is very slow. A few millimetres of height a year and a fraction of a millimetre of diameter is typical, so a branch thick enough to make a bead of any size represents decades and a harvestable colony often a century or more. Colonies reach reproductive maturity only after several years.

Those two numbers together are the entire management problem: the resource regenerates on a timescale of human generations, and it is harvested on a timescale of boat trips.

Red coral has carried protective meaning across an unusually wide range of cultures for an unusually long time. Roman children were given coral amulets against harm; Renaissance paintings of the Christ child put a coral branch around his neck for the same reason; it is one of the nine sacred substances of Hindu tradition and one of the seven treasures of Buddhism, and Tibetan and Mongolian jewellery uses it heavily.

Torre del Greco, near Naples, has been the centre of the working trade since the eighteenth century and still is, which is a rare survival of a single town holding a craft monopoly into the present.

The consequence of that breadth of demand is that coral has been harvested continuously in the Mediterranean for something over two thousand years, which is why the shallow, accessible beds have been gone since antiquity.

Of everything gathered from the sea for ornament, this is the one where the honest account is least comfortable.

Mediterranean Corallium rubrum populations are a fraction of their historical size and structure. The large old colonies are essentially gone; what remains is small, young and harvested before it reaches the size it once reached, which is the classic signature of a fishery that has been serially depleted rather than managed.

The Pacific Corallium beds that supplied the twentieth-century trade followed the pattern faster. Discovery, a boom of a few years, collapse, move on — repeatedly, through the Emperor Seamounts and the Hawaiian and Taiwanese grounds — and much of the historical harvesting was done by dredges and tangle nets that destroyed the whole benthic community to take the coral out of it.

Corallium species are listed on CITES Appendix III, which requires export documentation but is a weaker instrument than the Appendix II listing that has been proposed and defeated more than once. Selective harvesting by submersible and rotation of beds are real improvements over dredging; whether they are sufficient against a century-scale growth rate is not established.

A reader deciding what to buy is entitled to the summary: this is a material whose supply cannot respond to demand within a human lifetime.