Russian Gemstones Encyclopedia

Vladimir Bukanov. Russian Gemstones Encyclopedia



CORAL (Koralle—Corail— оралл), from Gk. “coralion”, which means “that one, which gets harden in a hand”; or Arabic “goral” – an amulet stone. This name has been known since antiquity. This is a bio-mineral formation of the group of hydrogenous jewelry-ornamental materials. Here also fossil corals and belemnites are described.

Composition & Properties – CaCO3 with the admixture of MgCO3, and for gorgonarium black and blue corals with admixture of horn material and conchialine. In olden coral colonies the content of MgCO3 can reach 40%. Hardness 3.5-4. Density 2.6-2.7, (for black coral – density 1.3). Dull, pearl or glass luster. The material of corals is fine-grained calcareous skeleton of coral polyps – Sea Coelenterate. They know 27 kinds of corals available for processing, seven of them are used in jewelry. Their calcareous skeletons are composed mainly with calcite, more rarely with aragonite. Among the main types of noble corals, we should mention: six-radial gorgonarium – red corals and white corals, eight-radial Madrepore – horn corals and a rare type of solar corals. Red corals are found mainly in warm seas, at the depth of 30-500 m., in the form of dendrite bushes, growing with the speed about 75 mm per year. The height of their treelike constructions reaches 40 cm. with the ranches up to 4 cm. in diameter. The biggest weight of a colony of red coral reaches 40 kg. They grow on the stony ground and don’t form reefs. Another type of eight-radial Madrepore horn corals is represented with bigger tree-like colonies up to 3 m. high with the trunks up to 25 cm. in diameter. The depth of their habitation can reach 1000 m.; the speed of growing is rather higher – up to 5 cm. per year. They are reef-forming corals. Among them, of the greatest interest is black coral, Hawaiian gold coral and Philippine golden coral. In the fresh state they are soft, after processing they become thermoplastic. Of the type of solar corals there is porous blue coral called so because of the cobalt-blue color of its calcareous skeleton.

Color of coral depends on the content of organic substance, the level of light during its growing and of admixtures, received at the stage of its growing. Iron admixtures color coral rose, red, orange and brown; manganese – gray. The color of black coral depends on the presence of the horn substance conchialine. There are also yellow, yellow-brown corals; the rarest are light blue corals. Dispersion of color is usually even, but it can be sonar, spotty or stripped. Japanese coral possess a skeleton of milky-white color with red core; among Mediterranean corals there were black samples with red core. Sometimes, such corals have oblong black hollows – tiger corals. Corals are non-transparent but translucent at the edges. The best gem-quality material is placed closer to the points of growing – the tips of coral branches. According the quality, they classify the following varieties: African coral – of high quality with straight, rather thick and moderate divaricated trunk; Algerian coral – of rather low quality because of ingrown sea organisms; Sicilian coral – of good quality but difficult in processing. This coral has been being processed for long on Sicily, near Shakka – Shakka coral. Spanish coral is of middle and low quality, crimson-red, it has several branches on one trunk. Hawaiian gold coral is of good quality, bright yellow-brown. On the basal slabs it displays zones of different thickness of color. As trade names for the quality of corals from the Mediterranean Sea, they use the following: Sardinian coral – a hard material of exclusively high quality, Italian coral – of good quality and different colors from white to red, as well as orange – sciacca coral, and worse in quality Sicilian and Algerian corals. In Japan, there are the following varieties of corals: rose-red coral under the trade names rose coral and angel skin coral, middle bright red one – Sazuma coral, orange one – Scotch coral, light orange, salmon color one – magai coral from Japan, spotty white and red one – Japanese coral, bright red of perfect quality – moro coral and extracted near Okinawa – coral sango as well as extracted in the Toso Bay, one of middle quality – Toso coral. In the world practice, they widely use an Italian scale of colors of corals: white – “bianco”, fresh-rose – “pelle d’angelo”, pale rose – “rose pallida”, bright rose – “rose viva”, orange-red – “secondo coloro”, dark red – “roso scuro”, very dark red – “arciscuro carbonetto”. In Italy, they classify the following sorts: I-sort “capo testa” – selected pieces of pale rose coral; II-sort “barbaresco” – common coral with knotty branches; III-sort “terraglia” – brownish to dark brownish, corroded by other organisms, dead fine branches and roots of coral; IV-sort “raspatelli” – brown-black, divaricated horn coral in wastes.

Locality. Corals are spread along the western coast of the Mediterranean Sea, in the Red Sea; in Atlantic they are along the coasts of Africa, Spain, France, the Canary and Bermuda Iss., Cuba, the U.S.A. and in the south of Ireland; in the Indian Ocean – along the coasts of Africa, in the Persian Gulf, near India, Malaysia, Northern Australia; in the Pacific Ocean – near the coasts of China, Japan, Hawaii, and in the north of Pacific – near the Aleut Iss. Near the Algerian coast, they extract the best red corals, and near the coast of Western Africa – a rare horny blue coral – akori. It is also found in the Pacific Ocean, at the atoll Funafuti. In Tunisia, they extract about 20 tons of corals per year, 75% of them are exported to Italy. In the Red Sea and in the Indian Ocean, near the Sokotra and Mauritius Iss., there are black corals. They are also found near the coasts of India, China, Cuba, Nicaragua, near the northern coast of Australia and around Malaysia. In the Pacific Ocean, on Hawaii, resources of noble corals are valued in 70 tons. Their extraction is regulated with a quota. In Japan, the most part of corals are extracted from the Tosa Bay, Shikoku Is., including boke – pale yellow coral of the color of quince. Yellow-brown corals are spread near the Aleut Iss., in the northern part of the Pacific Ocean.

Beside modern corals, we should mention their ancient analogues – fossil corals and fossil mollusks – belemnites, which are used sometimes in adornments or as amulets. Fossil corals were widespread in all the post-Cambrian ages. They are often found in limestone layers. In slightly polished form, they look like agate. Such petrified coral or agatized coral from the region of Petoskey, states of Michigan, the U.S.A., they call Petoskey stone. It is of some interest as an ornamental stone. Similar pseudomorphs on corals with a star pattern of their skeletons on the basal slabs are known in all the Florida and in the region of the Panama Channel. They are widespread in many mountainous regions of Russia, but they were still not really valued as ornamental stones.

Belemnites are fossil cephalopodan mollusks. The same name is used for petrified parts of their skeletons – the inner carbonate shell in the shape of rostrum, resembling a finger. They were one of the first amulets and adornments of ancient humans. Of the biggest interest are belemnites from Australia, sometimes completely replaced with noble opal.

Synonyms. Red: ~ coral: angelic ~, blood ~, oxblood ~, momo ~, from Taiwan; precious ~, red ~, spotty ~. | Blood flower | Bloody foam | Sardegna, Ital.

Black – Accabar, Arabic | ~ coral: black ~, horn ~, king’s ~, tiger ~, violet ~ | Giogetto, Ital. | Scorpion stone.

Yellow – Boke, Jap. | Gold coral | Hawaiian coral | Tunisian coral.

Fossil coral – Coral agate | Petoskey agate | Alveolite, Germ. | Beekite.

Belemnite from Gk. “belemnon” – a thunder arrow, after the shape of its skeleton, or devil finger.

Quality improvement. To restore faded colors of corals they are put for a short time into a solution of hydrogen peroxide. With this method they also make more valuable rose corals from red ones. Besides, discoloration of corals is produced with melted wax or turpentine oil. To transform or to strengthen their color they can be saturated with paints. Usually, they use this method for the improvement of white coral. Philippine golden coral is processed with an acid or hydrogen peroxide for the imitation of more valuable Hawaiian golden coral. From plain black coral with the method of annealing they receive dark brown or blackish burnt coral – bruciato.

Treatment. Corals were among the first materials used for the manufacturing of adornments. They were popular in Ancient Egypt and in the countries of Mesopotamia. In Europe, they were discovered in burials dated to the 10th millennium B.C. The most intensive coral trade was held in the epoch of the Crusaders; the center of it was Venice. In the 16th cent., the main market of coral produce was transferred to France. After the Great French revolution it got into decay, and the priority came to Italy – to Torro del Greco, at the sole of Vesuvius. In the period of the most active usage of corals in jewelry, from 1879 till 1884, they produced the weight of 6,500 tons of coral goods. Nowadays, centers of coral processing are Japan, Taiwan, Siangan (Hong Kong) and India, because the resources of corals in the Mediterranean Sea were almost completely worked out. From white corals they make cult objects for churches, home utensils and adornments. Red corals are used for beads – so called Red Sea pearls, chains, gems, bracelets, rosaries, cabochons and small forms. Recently, unprocessed branches of corals come in fashion; in Italy they are called cornetti. Coral branches in the frame of gemstones are rather picturesque in necklaces and brooches. They incrust hilts and cult objects with coral, for instance, icon frames, book-covers for Church and others. In Moslem countries, they make belts, composed with unprocessed pieces of coral. From black coral they make funeral garlands and flowers, as well as rosaries, beads, bracelets and hoops, handles of walking-sticks and hilts. It is especially high evaluated in the countries of the East. Blue coral – akori is polished with big difficulties, because it is porous. In Europe, it appeared in the 17th cent.; it came from the western coast of Africa. From its branches they made elegant female hair adornments. Nowadays, they can be found only in the property of aboriginal people of the Samoa Iss.

Peoples of the Caucasus, Middle Asia and Eastern Europe widely used corals in adornments and decoration of armaments. In the 13-14ss cent., in Russia, corals were processed in beads of different shapes, insets for jewelries and even in balls-buttons – “kinglets”. In the Museums of the Moscow Kremlin, there are works form coral by Ital. masters. There is a carved brooch from red coral, 6x4 cm. in size, in the shape of a basket of flowers, in combination with pearls, a coral bracelet mounted in gold and enamel, 21 cm. long, earrings and a ring with insets from carved coral in the shape of heads of Bacchants and Bacchus. A coral pencil with a carved portrait of Napoleon, mounted in gold, 9.3 cm. long. In the State Hermitage, SPb., among works by Ital. masters, there is a coral set, containing a bracelet, a necklace, a comb, a brooch, ear rings and a pair of hair-pins. Each of these objects consists of fruits and leaves, collected from corals with a central bead, 1.2 cm. in diameter. Especially beautiful are coral rosaries with a silver icon and a filigree cross. A carved image of a woman at the chariot, horsed in four mounts, is very elegant. The height of this object is 27 cm, all the figures are made from red coral. Unique in mastery is a chalcedony stamp with a coral handle, as well as a silver dish with artistic insets from red coral or a coral figure of a harlequin richly decorated with gold and gemstones or a cameo on coral with a portrait of Richelieu, 4x2.9 cm. in size.

Among unique works from of art from other collections, we should mention a work by a Parisian jeweler of the 18th cent., who made a vessel in the shape of a temple, 50 cm. high, with an ornament in coral. French jewelers got a real mastery in carving on coral. From coral cameos and carved heads they composed marvelous necklaces, parures (Fr.) and bracelets. Carved corals insets decorated lockets, pins, clocks and watches. In 1878 at the Paris exhibition, they exhibited the chess from colored corals with figures in the shape of Saracens and Crusaders, which were evaluated in Fr 10,000. In the Grün. Gew., Dresden, there are samples of using of coral in tableware. Among unique works, of art, we should mention an altar-piece table, 32 cm. high, richly decorated with gemstones and carved figures from coral, as well as a figure of “Daphna”, exquisitely represented with a branch red coral, when it was turning into a tree. In 1880 at the International exhibition in Berlin, they exhibited a coral necklace, which was evaluated in Fr 10,000. Wonderful artistic pieces from corals are made in China and Japan. There are schools of tracery carving on corals. A sample of such work of art is a Chinese dragon from coral, 143x110 mm. in size, in the collection of the State Hermitage, SPb.

Legends. According a Greek legend, coral appeared from drops of blood of Medusa Gorgona, decapitated by Perseus. That’s why red coral is called gorgonarium coral. In Ancient Greece, rose coral was a symbol of immortality and happiness, and in Europe, it was a symbol of modesty. As a talisman, coral improves memory. As an amulet, it supposed to give a protection against poison and witchcraft, shortness of breath and loss of appetite. It was used in healing of eyes and teeth, headache and other diseases. Astrologers believe coral is a lucky stone: red coral – for those, who were born under the sign of Pisces, Aries and Scorpio; rose coral – Taurus, Cancer and Libra. Coral is regarded as a national stone in such countries of the Mediterranean as Algeria, Italy, Morocco and Serbia. In the U.S.A., coral is an official symbol of the Florida State, and fossil coral – of the Michigan State.

Synthesis. Synthetic coral, or Gilson’s coral was received artificially by the firm Gilson from calcite powder in 1972. It is manufactured in twelve color varieties. Its properties are close to the same of natural coral, except greater porosity and less density – 2.6-2.7. In the content and outer characteristics, synthetic coral resembles natural one, but it contains the admixture of SiO2 and of an artificial material, that’s why it is classified as an imitation. Its market price is in 7-10 times less than that one of natural coral.

Similarity & Imitations. Corals in cabochon look like carnelian, rhodonite and spessartine. As an imitation, they use reconstituted coral from pressed chips of corals, gypsum, horn, bone, marble, plastic, rubber, coroso nut, glass and porcelain. As a material for artificial imitation, they use also barium phosphate with binding components. Coral itself can be used sometimes as an imitation of pearls, its African trade name is olivet pearls.