CaCo3 or calcium carbonate in the form of calcite, is the main constituent of calcareous corals: minor constituents are MgCo3. or magnesium carbonate and proteinaceous organic substances, which act as binding agents. A horny, proteinaceous substance whose composition varies according to the species is, on the other hand. the basic constituent of horny corals. Most of the coral used since antiquity as an ornamental material comes from the calcareous skeletons of colonies of marine organisms of the phylum Cnidaria, order Corgonacea, genus Corallium.

The most famous of these organisms is Corallium rubrum, which lives in the waters of the Mediterranean and, despite its name, provides not only red. but orange, pink, and white coral. Similar to this are Corallium eiatius, C. japonicum. and C. secundum, which mainly live off the coasts of Japan, China, lndochina, the Philippines, and other archipelagos of the Indian and Pacific Oceans. Other corals are
occasionally used for ornamental purposes as well. They are the calcareous skeleton of Heiiopora coerulea of the order Coenotnecaiia, which produces what is known as blue coral, and the horny, proteinaceous skeleton of corals of the orders Gorgonacea, Zoanthinaria, and Anthipatharia, which supply so-called black coral.

Crystal system Calcite, which is the main component of calcareous corals, crystallizes in the trigonal system. The proteinaceous substances of the horny corals are non-crystalline.

Appearance The skeletons of corals vary in color: from bright to dark red, slightly orange—red, pink, and white, for Corallium rubrum of the Mediterranean; from red to orange red to orange pink with areas of white for C. japonicum; and from medium to deep pink, sometimes with alternate, concentric layers of lighter and darker color and a whitish portion corresponding to the axis for C. elatius; C. secundum has alternate, wavy lines of light and dark pink; while coral skeletons of the genera Gorgonia, Euniceila, Gerardia, and Parantipathes are blackish on the inside, with slightly more translucent, brown areas; and Heiiopora coerulea is cold, almost gray—blue. The outer surface is compact and fairly regular, opaque, in most cases, except for blue coral, which is very porous, and the horny, black coral of the genus Parantipathes, which has numerous small surface protuberances, like gooseflesh.

Characteristic features are differences in transiucency. or the arrangement of the streaks of color crosswise or lengthwise to the branches. On cross sections. observable by cutting through a branch. a concentric ring structure is always visible to some extent, sometimes with very faint rays cutting across it; this is very similar to the growth rings and medullary rays of tree trunks. Sometimes. coral also has a different-colored marrow or medulla (generally white). or an axial canal. All this produces a series of longitudinal alignments. closely resembling the grain of wood. which are visible on polished, outer surfaces or on longitudinal sections. Both cross and longitudinal sections of the ramifications display features very similar to those observed if a branching tree is cut into sections. These features are strictly determined by the organic structure and growth pattern of coral colonies and their skeletons. They also explain why, up to the early eighteenth century. coral was believed to be a vegetable, a type of small, submarine tree.

Physical properties These are otton hard to measure, except for the density. which shows clear differences, especially between calcareous and horny corals. For the tirst, the density is usually between 2.60 and 2.70 g/cm3, but may be even lower. starting from 2.35 g/cma, when the coral contains large amounts of the organic Componentwhich binds the inorganic parts together. Blue coral alsohas a low density of about 2.46 gfcm-"_ The refractive indices. which are hard to measure, are about fie" 1.49. nw, 1.65. thus being approximately the same as for calcite. At 2.5-4, the hardness is slightly higher than that of calcite. The organic substance helps give the coral good tenacity, enabling it to be fashioned and polished to a high degree. For horny corals. the density varies from 1.33 to 1.42 g/cm“ or thereabouts. and the retractive index ranges from 1.54 to 1.56. Although they only have a hardness of 2.5—3, these corals take an equally good polish. They also have a certain degree of elasticity and can be heated and
bent into bangles.

Genesis in all cases, coral consists of the branching skeletons of animals which live in colonies planted on the seabed at depths varying from tens to hundreds of meters. They are typical of warmish to very warm seas.

Occurrence Banks of coral are tound in the Mediterranean, along the coasts of China. Vietnam. and Japan; near the Philippines, along some of the many Pacific archipelagos: and along parts of the African coastline. Coral colonies occupy large areas especially in the Pacific, but also near the coast ofSouth Africa. in the Red Sea. and to the east of Australia. These latter colonies, however, consist ot madrepore. which has little in common with the corals used as ornaments.

Red Coral
The oldest known findings of red coral date from the Mesopotamian civilization, i.e. from about 3000 ec. For centuries, this was the coral par excellence, and at the time of Pliny the Elder it was apparently much appreciated in India. even more than in Europe. The name is derived from the Latin coraliium, related to the Greek koralilon.

Appearance The red coral from the Mediterranean (Corailium rubrum) has very faint concentric rings. It is easier to see the longitudinal structures. The type from the seas near Japan has more clearly visible organic structures. The thin branches were and still are polished. pierced. and threaded, unaltered. into necklaces. Larger pieces are cut into spherical or taceted necklace beads. pear shapes for pendant jewelry. or cabochons. This coral is very compact and easily acquires a good polish. although this may deteriorate in time as the material is not very durable. It is also used for carved pieces and small fig-urines, in both oriental and western art styles. The most highly prized varieties of coral are those that are a uniform. strong, bright red. Specimens that are too light or too dark, or have an orange tinge or unevenly distributed; color. are less valuable. Some basically red Japanese corals have a white axial portion. This is, of course. regarded as a defect. where it is not eliminated in the cutting
process.

Distinctive features Two basic facts must be remembered in distinguishing coral from its imitations:

1) The specimen should have the organic structure characteristic of coral;
2) On contact with a drop of hydrochloric acid (the readily available muriatic acid), the piece should display the strong effervescence characteristic of calcite. The most frequent imitations are of glass. These have longitudinal striations similar to those of coral. but do not react to hydrochloric acid. Another common simulant is made from a compact, artificial agglomerate mainly of calcite. which lacl-as any trace of organic structure. As mentioned, the hardness is about 3.5, therefore well below that of glass. The organic substance in coral also gives it very good tenacity, tar superior to that of the constituent mineral alone. Because of its organic content, the density varies quite widely, but is usually about 2.60-2.66 g/cm3.

Occurrence Red coral is gathered in the Mediterranean (mainly near Sardinia and Sicily), the Eastern Mediterranean, the Flecl Sea, and the seas around Japan.

Value When made into polished and/or faceted necklace beads. it has roughly the same value as that of certain prized ornamental stones, such as good quality turquoise and lapis lazuli, The color, homogeneity of each piece (individually and as part of a necklace) and polish are very important. When the coral is made into carved pieces and figurines, the quality of the work is obviously very important. Furthermore, it should be remembered that large pieces are hard to find.

Stimulants and synthetics Very small pieces of coral or branching twigs are imitated by special glass which simulates the typical longitudinal structures; but clearly fused surfaces, the presence of air bubbles, and immunity to at tack from hydrochloric acid uncover these imitations quite easily, especially if they are viewed through a lens or binocular microscope. Nowadays, a coral-colored agglomerate consisting mainly otcalcite and called synthetic coral is also manufactured. This reacts to hydrochloric acid in the same way as true coral but lacks the characteristic structures of the organic version. As with other gems and ornamental materials of organic origin, the term synthetic is not very meaningful.

Pink (and white) coral
Not often seen on the market a few decades ago, pink coral is now widely available; large quantities of it come from the Orient.

Appearance The Mediterranean type fa special variety of the more common red coral) is very compact and. like red coral, takes a good polish, with barely visible organic structures and a fairly uniform, soft pink to white color. The oriental variety often has more clearly visible organic structures, sometimes emphasized by the presence of a white center or concentric color zoning. it is often very pale, with shaded areas or patches of pink or orange pink. On other occasions it has concentric zones of color from very bright pink to light pink or whitish; but it may be a beautiful uniform pink, very similar to Mediterranean coral. Sometimes, the rings of the trunk are genuine discontinuities or cracks and there may be other extensive radial or variously oriented cracks. making the whole structure more brittle and therefore less valuable. Costliest of all are the most compact, easily polished varieties, without cracks or cavities, of a perfectly uniform soft pink color, without any trace of orange. When pink coral has all these characteristics, combined with an antique pink color, and with the merest hint of violet, it is known as pelle d'angelo or "angel's skin." It is meaningless to describe patchily colored coral as “pelle d'angelo type" or "part pelle d'angelo, " as the very existence of patches or discontinuities rules out such a definition. The inferior varieties often have poor polish, cracks, and, as a rule, some artificial color.

Objects manufactured from pink coral include polished, spherical necklace beads. roughly carved but rarely faceted pieces. necklaces, pendants, cabochons, and other items of jewelry, and figurines.
Distinctive features As with red coral, the most important distinctive features are the typical organic structures (clearly visible in evenly colored corals, but much less apparent in the others) and reaction to hydrochloric acid.

Minute examination is necessary to distinguish it from the pink shell used for the same purpose. The structure of the latter is different, consisting of almost flat or slightly curved parallel Layers, never concentric rings; but the reaction to hydrochloric acid is identical. With pink coral, it is very important to establish whether the color is natural or whether, as often happens nowadays, the color of almost white coral
has been heightened by the use of dyes.

Traces of dye may be visible in small, superficial cavities, or one may be able to see, by splitting one bead of a necklace, that the outer surface and that of a preexisting crack are more deeply colored than the newly fractured one.

These are the main methods of detecting the presence of artificial colorants.

Occurrence Very limited quantities of pink coral come from the Mediterranean; considerable quantities, although mainly of inferior quality, come from the-Far East, especiallv -.lflD8I'1- The pelle d'angelo variety, which is not commflfl. may come from the Mediterranean, the Gulf of Chihli (in China), or from Japan.

Value Good quality pink coral of a uniform and attractive color is worth at least as much as red coral. Most of the pink coral on the market is, however, of inferior quality and has been artificially colored. It is therefore much less valuable and is worth perhaps a quarter of the red variety,

White coral also has quite a low value. given its poor ornamental qualities.

Stimulants As already mentioned, pink coral can be imitated by similarly colored shells, which, apart from having a different structure, have a slightly higher density of about 2.85 9/cm“ {compared with 2.63-2.70 g/cm“ for pink coral). But the main problem with this type of coral is the common practice of using dyes to improve coral that is mainly white, contains a few streaks of pale color. or is distinctly patchy. In some cases it is difficult and costly to detect this type of fraud. Perhaps for this reason, pink coral has fallen sadly into disrepute. except for the better varieties obtained from reliable sources.

Blue Coral
This material, unusual in appearance and color, and of limited use, is obtained from the skeleton of Heliopora coerulea. a type of coral which lives mainly in the seas around the Philippines.

Appearance The most striking characteristic of blue coral is its color, which is bright blue or gray blue, sometimes with zoning in the form of concentric circles or even horizontal stripes. The most readily observable organic structures are two sets of channels parallel to the axis: one set is thin and barely visible, though numerous. The other is more prominent, less numerous, and larger, with dark walls: these channels are placed at regular intervals between the other channels. Due to the cavities. marks, and discontinuities where the channels emerge, blue coral never takes a polish comparable to that of red coral.

Therefore, its use as an ornament is limited and mainly dependent on its color. It is made into spherical, cylindrical. spindle or barrel-shaped necklace beads, but does not lend itself to other uses.

Distinctive features The color and clearly visible organic structures make it quite easy to recognize.

Occurrence Blue coral comes mainly from the Philippines, where it is generally fashioned as well. Like other local materials such as black coral, shell. and porous white madrepore coral it is made into inexpensive necklaces.

Value Very low. partly because it is difficult to fashion and polish. It is essentially a curiosity on the western market.

Stimulants It is not imitated at present. probably because it is little known.

Black Coral
This consists of the skeletons cl polyp colonies, mainly or the genera Gorgonie, Eunicella, Gerardia, and Parantipathes; but unlike those that make up red, pink, and blue coral. these skeletal remains are oi a horny nature, not calcic.

Appearance The color is black, but sometimes has minute, short, brownish yellow, slightly translucent streaks. It can acquire quite good luster if polished, but this will be of a horny character, similar to that ot some plastics. It is used in cylindrical pieces which are drilled atong the axis orhorizontally to it, as necklace beads. It can be bent it heated and made into bangles. Cheap rings, carved items. and tigurines up to ten centimeters tall are also made from larger pieces.

Distinctive features It cut crossways to the axis, the characteristic concentric rings, like those of tree trunks. are visible. These sometimes have marked discontinuities between one and the other, almost as though they were becoming detached. Faint radial structures and thin longitudinal structures, yellowish brown in color and slightlytranslucent, may also be visible. Sometimes, these limited areas of yellow-brown translucency show small protuberances on the underlying sui-lace, as would have been present on the outer surface had it not been polished (in black coral of the genus Parantipathes). It is no use testing black coral with hydrochloric acid, as it does not contain calcium carbonate; but it should be remembered that the density is about 1 .36 Q/cm“, which is much lower than that ol red and pink corals.

Due to its oroteinaceous character. black coral emits a smell of burning horn if touched with a piece ot red-hot wire. It is warm to the touch, like plastic. has a relatively low hardness of between 2 and 3. and is slightly elastic.

Occurrence It is gathered near the Hawaiian islands, in the Great Barrier Fleet of Australia, in the Fled Sea. on the west coast of Africa, the Antilles and. occasionally. in the Mediterranean.

Value Distinctly low, much lower than that ot the main types of coral. As an ornament it is characteristic of many different cultures, but in the West it is mainly regarded as a curiosity, although appreciable quantities were seen on themarket a tew years ago.

Sirnulants It does not appear, at present, to be imitated.