Not often seen on the market a few decades ago, pink coral is now widely available; large quantities of it come from the Orient.
The Mediterranean type (a 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 pede 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.
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 neck-lace, 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.
Very limited quantities of pink coral come from the Mediterranean; considerable quantities, although mainly of inferior quality, come from the Far East, especially Japan. The pefle d'angelo variety, which is not common, may come from the Mediterranean, the Gulf of Chihli (in China), or from Japan.
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.
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 g/cm3 (compared with 2.63-2.70 g/cm3 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.
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.
The most striking characteristic of blue coral is its color, which is bright blue or gray blue, some-times 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 vollend itself to other uses.
The color and clearly visible organic structures make it quite easy to recognize.
Blue coral comes mainly from the Philip-pines, 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.
Very low, partly because it is difficult to fashion and polish. It is essentially a curiosity on the western market.
It is not imitated at present, probably because it is little known.
This consists of the skeletons of polyp colonies, mainly of the genera Gorgonia, EuniceIla, Gerardia, and Parantipathes; but unlike those that make up red, pink, and blue coral, these skeletal remains are of a horny nature, not calcic.
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 beat a horny character, similar to that of some plastics. It is used in cylindrical pieces which are drilled along the axis or horizontally to it, as necklace beads. It can be bent it heated and made into bangles. Cheap rings, carved items, and figurines up to ten centimeters tall are also made from larger pieces.
If 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 slightly translucent, may also be visible. Sometimes, these limited areas of yellow-brown translucency show small protuberances on the underlying surface, 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 g/cm, which is much lower than that of red and pink corals. Due to its proteinaceous character, black coral emits a smell of burning horn if touched with a piece of 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.
It is gathered near the Hawaiian islands, in the Great Barrier Reef of Australia, in the Red Sea, on the west coast of Africa the Antilles and, occasionally, in the Mediterranean.
Distinctly low, much lower than that of 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 the market a few years ago.
It does not appear, at present, to be imitated.
Writer – Curzio Cipriani and Alessandro Borelli