A Background to Corals

A BACKGROUND TO CORALS

The world of Organic Gems spans the spectrum from the largest mammals (whales, elephants) to some of the smallest animals, namely coral polyps. They are related to sea anemones. Note: Corals are animals, not plants. In spite of some corals occasionally being misleadingly referred to as ‘vegetable coral’, there is no such thing as a coral plant.

Corals grow in various parts of the world, but most prefer warm seas. However, the depth of the water affects its temperature and some corals grow in deep water where the seas are cooler and there is less light.

Polyps grow in colonies, attached to a communal ‘scaffolding’ or hard core made mostly of calcium carbonate. (In the case of black and golden corals the hard core is made of a horny material related to keratin, which is a fibrous protein very prevalent in the animal kingdom.) In living coral, the hard core is covered by ‘flesh’ which consists mainly of tiny polyps connected to each other by living tissue. In most corals the tissue also contains algae called zoolxanthellae with which the corals live in a symbiotic relationship. It is the algae that give the living corals their wonderful colours. In only a few of them is the calcium carbonate scaffolding coloured.

Corallium rubrum, flesh removed, showing red 'scaffolding'

In the case of gem corals, the core is coloured, and it is this that jewellers, gemmologists, and those affiliated to the trade call ‘coral’.

In the gem trade the common corals are: ’Precious coral’ (Corallium corals) which occur in colours ranging from white through pinks to deep red (varieties include Corallium rubrum; C. secundum; C. japonicum, etc.); blue coral (Heliopora coerulea) which is a soft, grey blue; black and golden corals (of various species) which are black and golden as the names suggest, and red soft coral (Melithaea ochracea) which is red. Only bamboo coral (of the Isis family) is brown, but this is almost always bleached and dyed when worked.

The word ‘coral’ is, at best, a loose term. It originally referred to any animal with a skeleton, that grew attached to the sea floor. It is further complicated by the fact that coral taxa keep changing, due in part to research uncovering new facts, new species, and so forth. The Corallium corals have recently been divided into new groups, which is of importance to the gem trade as some species are protected. The new names, however, are not yet widely known or used so will not be used here.

For the sake of simplicity the word ‘coral’ will, for the remainder of this article, refer to the part of the animal which is of interest to the jeweller, namely the hard, ‘scaffolding’ core.

CORALLIUM CORALS

As already stated, Corallium corals are found in a range of colours from white, to a deep red which is often referred to as ‘ox blood’. (A pink variety is sometimes called ‘angel skin’.)

The best known colour, and the one which has been used (and much prized) in the decorative arts for many centuries, is a deep, orange-pink hue, which has given the name to the colour 'coral'.

Corallium corals grow in colonies in deep waters, in a form resembling small trees. The material is identifiable by its structure of fine ridges which run the length of the ‘branches’ on unworked material, and are ¼ to ½ mm apart. When polished the structure is still visible as fine lines. They are not just on the surface, but penetrate the material the whole way through. In cross-section they resemble the pattern of a spider’s web.

Left: Corallium coral cabochon, showing the structure of fine lines. Right: The structure in cross-section, seen by transmitted light..

Corallium corals have been used to make everything from simple beads to very intricate carvings. Originally fished in the Mediterranean, most of the material now comes from the Philippines and around Japan.

It is occasionally dyed to enhance its colour (some colours are more valuable than others), and when mounted in jewellery it can be impossible to detect the dye without using laboratory tests. It may sometimes be visible if it has collected in small fissures.

The paler the colour of the Corallium coral, the harder it may be to see the identifying structure. Thus, a cabochon carved from pink shell imitating one carved from pink Corallium can pose a real problem. Plastic imitations are not difficult to spot, though, as they tend to be light and warm to the touch, whereas Corallium corals -- which consist mainly of calcium carbonate -- are heavier and cold.

Left: Imitation of Corallium rubrum, lacking structure. Right: Dyed Corallium rubrum, with dye collected in fissures.

BAMBOO CORAL

Bamboo coral gets its name from its growth pattern, which somewhat resembles bamboo because the coral ‘branches’ – which are made predominantly of calcium carbonate -- are interspersed with nodes of horny material called gorgonin.

Bamboo coral is a much coarser and heavier material than Corallium coral, though it, too, has a structure of fine lines. In bamboo coral they are 1 mm apart, and do not penetrate to the centre, which lacks structure and can have a 'gritty' appearance/

Left: Bamboo coral, (left) dyed, and (right) bleached. Right: Bamboo coral in cross-section.

Usually used in the form of inexpensive beads (carved or plain), or as carvings, bamboo coral is almost always dyed, mostly red but also orange, though it can be dyed any colour. The dye tends to collect in cracks (of which there can be many) and especially in the gorgonin sections of the material.

Although it could be used as an imitation of Corallium coral, it does not lend itself to fine carving and any item made is limited by the size of the internodes.

Left: Dyed bamboo coral, slices strung as a necklace and sections carved for a necklace. Right: Dye collected in horny internodes.

BLUE CORAL

Blue coral is more rarely seen today as it is now a protected species.

Blue coral, Heliopora coerulea, is found in the Indo-Pacific region where it grows in reefs (the only gem coral to do so), and forms various shapes. It can have a somewhat spongy appearance due to the holes – some small and some tiny – in its structure, but it is in fact hard and rigid.

Good quality blue coral is more dense than poorer qualities, which may be impregnated with resin to stabilize them. A blue dye is sometimes added to enhance the colour. Resin (and dye) may be apparent when it accumulates in the holes. The material may further be capped with clear plastic to give extra stability and be more economical, though good quality coral has no need of these additions and takes a good polish, resulting in a satin lustre.

Left: Blue coral, beads and rough, undyed. Right: Detail of a dyed piece, showing dye collected in the small holes.

Although it can be carved, blue coral is more usually used as beads or simple shapes as the presence of the holes make intricate carving impossible.

The outer surfaces of natural blue coral can fade if exposed to daylight for a prolonged period of time. It is likely that the same would apply to the worked material.

RED SOFT CORAL

Some years ago Melithaea ochracea -- red soft coral (often incorrectly called ‘sponge coral’) -- was almost unknown, but as other corals have become more scarce it has become quite common.

Red soft coral gets its name from the family of corals to which it belongs (see note below). It is not soft, but is hard and very abrasive in its natural state, growing in the shape of small, sturdy trees. It lacks any of the striations seen in Corallium or bamboo corals but has a mass of small holes similar to blue coral.

It is usually necessary to impregnate it with resin -- often with the addition of an enhancing red dye – to stabilize the material. This also makes it more comfortable to wear. The result can be a satin finish.

Left: Melithaea in rough form. Centre: Dyed beads, and disc impregnated with polymer and polished but not dyed,

showing natural veins in material. Right: Melithaea chips embedded in polymer, and sold as red coral.

Red soft coral is usually encountered as beads. It does not take intricate work, though it may be found as simple carvings.

Following the appearance of this material on the market came a material sold as ‘red coral’ but which, on close inspection, does not look natural. Laboratory tests show that small chips of red soft coral (which may or may not have been dyed), are embedded in red polyester resin, which is then moulded, cut and polished to a desired shape.

Note: The family of ‘Soft corals’ is so-called because the calcium carbonate particles forming the ‘skeleton’ laid down by coral polyps in this family do not always join up forming a solid structure, and instead may evolve into a slightly flexible coral. However the red soft coral used in the gem trade is totally rigid.

BLACK AND GOLDEN CORALS

Alone amongst the gem corals, black and golden corals do not consist of calcium carbonate, but of a horny material akin to keratin and the material that makes up the nodes in bamboo coral.

Black coral can be either a Gorgonian or an Antipatharian coral. When worked it can be almost impossible to see from which species an item derives. Some may show growth rings in cross-section, which may be apparent on the worked material. Others are known for their spiny characteristics. The spines are polished off the outer surfaces and can leave a dimpled structure.

Left: Black Gorgonian coral showing growth rings. Right: Black Antipatharian

coral bleached in hydrogen peroxide to imitate golden coral.

Golden coral belongs to the same family as one of the black corals – Antipatharia – and also has the spines for which the family is known.

As it consists of a horny, keratinous material related to horn, claws, fur, nails and hair, etc.), it is possible to bleach black coral in strong hydrogen peroxide solution and turn it golden in colour. Without cutting the material in half it can be very difficult to tell whether it is natural golden or bleached black coral, though true golden coral has a more delicate colour.

Today black coral is also being turned golden by a process thought to involve chemicals and heat (the exact process is unknown). The result is less convincing than the bleached variety as it has the appearance of a coloured coating.

It is possible to slightly alter the shape of black and golden corals through the application of heat and pressure – a process impossible with the corals consisting of calcium carbonate -- and thus bend them into shape for use as bangles.

Black corals grow in the Red Sea, parts of the Far East and around New Zealand. They may consist of branched growths or of a single, long, coiled strand. Black coral has been carved but is usually found in the form of simple beads or bangles. Some black coral takes a high polish. The colour does not appear to fade.

TESTS and FAKES

The best test is, as always with organics, visual. There are few imitations of anything but Corallium coral, which is best imitated by shell.

Shell also has a structure of striations, but these are much finer than those of any coral. (See 'A Background to Shell.) No chemical tests are appropriate as they are destructive, and as shell and coral both consist mainly of calcium carbonate they would react in the same way.

Glass or plastic imitations lack any form of structure, and plastic tends to be light and warm to the touch.

One early plastic was used with some success as a simulant of Corallium rubrum. This was cellulose nitrate (‘celluloid’). If worn as a brooch and not handled (when its weight and lack of coldness would become apparent), it made a convincing copy. Close inspection would, however, reveal the lack of structure, and possibly also marks from the moulding process.

Brooch of cellulose nitrate (celluloid) imitating Corallium rubrum.

CONSERVATION STATUS

We do not emotionally equate the use of coral with that of ivory, but corals play a significant role in marine ecosystems and need to be protected, and failure to do so could eventually result in a total ban on the sale of the beautiful material. Already today there are many traders who will not sell it.

Corals grow extremely slowly and are very sensitive to changes in their environment such as temperature and pollution. The corals growing at greater depths, e.g. Corallium corals, are more protected from pollution than those growing in shallow waters, and are also more protected from rising sea temperatures or altered ocean acidity caused by climate change.

Rising ocean temperatures can result in ‘coral bleaching’ -- a phenomenon where large areas of coral reefs, which are in shallow waters, reject their vital algae and consequently die off, losing their coloured fleshy covering and leaving just the white skeletons. As stated, corals grow very slowly -- some at a rate of a few millimetres per annum – so it takes from ten to twenty years for a reef to regenerate. As coral bleaching is today happening more often it is becoming less likely that the corals can recover, hence the worry about the Great Barrier Reef off Australia’s North-East coast, and also the reefs around Belize. Being a reef coral, blue gem coral is affected by this.

Over-fishing has been largely to blame for the depletion of the coral beds world-wide, but today people are more aware and more care is being taken. The beds can also be damaged by actions such as dredging and fishing nets.

The Mediterranean, once rich in Corallium corals, is now somewhat depleted, and coral fishing is permitted there only under strict license and in certain areas at a time, thus allowing the corals to regrow in others. Today most Corallium comes from the Far East, where two species are listed by CITES.

Black and golden corals are also listed by CITES Appendix II, as is blue coral. As CITES listings change it is always advisable to check before buying coral, and only buy from a reputable dealer.