Monday, 20 May 2013

Coral An Gem Stone

Seabed with Corallium rubrumCaCo3, 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 Coralhum. 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 elatius, C. japonicum, and C. secundum, which mainly live off the coasts at Japan, China, Indochina, the Philippines, and other arch-occasionally of the Indian and Pacific Oceans. Other corals are occasionally used for ornamental purposes as well. They are the calcareous skeleton of Heliopora coerulea of the order Coenothecalia, which produces what is known as blue coral, and the horny, proteinaceous skeleton of corals of the orders Gorgonacea, Zoanthinaria, and Anthpatharia, which supply so-called black coral. 

Crystal system

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


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 Gorgonian, Eunicella, Gerardia, and Parantipathes are blackish on the inside, with slightly more translucent, brown areas; and Hehopera 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.

Figurine in red CoralCharacteristic features are differences in translucency, 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 medullar 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 obi served if a branching tree is cut into sections. This feature is 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 often hard to measure. Except for the density, which shows clear differences Especially between calcareous and horny corals For the first, the density is usually between 2.60 and 2.70 g/cm:', but may be even lower, starting from 2.35 g/cm3, when the coral contains large amounts of the organic component which binds the inorganic parts together. Blue coral also has a low density of about 2.46 9/cm3. The refractive in-dices, which are hard to measure, are about n4 1.49, rt. 1.65, thus being approximately the same as for calcite. Al 2.54, 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 9/cm3 or thereabouts, and the refractive 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. 


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. 


Banks of coral are found 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 of South Africa, in the Red Sea And to the east of Australia these latter colonies, however. Consist of madrepore, which has little in common with the corals used as ornaments.

 Skeleton of red coral Below Figurine in red coral Red Coral

The oldest known findings of red coral date from the Mesopotamian civilization, i.e. from about 3000 oc. 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 corallium, related to the Greek korallion.


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 faceted necklace beads, pear shapes for pendant jewelry, or cabochons. This coral is very compact and easily acquires a good polish, al-though 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 is 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 reach to hydrochloric acid. Another common stimulant is made from a compact, artificial agglomerate mainly of calcite, which lacks 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, far 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. 


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


Parure in red coral (1830) Musee des Arts Decoratifts ParisWhen 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.

Simulants 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 bin-ocular microscope. Nowadays coral colored agglomerate consisting mainly of calcite 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.

Writer – Curzio Cipriani & Alessandro Borelli

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