What is coral. A coral, unlike most other gemstones is actually the skeletal remains of marine polyps. The polyps are surrounded by a fleshy skin and excrete a carbonic substance from which the coral grows like trees and branches. This happens in the warm waters of the ocean at depths of around 10 to 500 feet where various coral species survive. In some places saltwater corals also exist.
Precious corals or red corals are called corallium rubrum, a word that is derived from a Latin word related to the Greek word Koralliom. The hard skeletons of the red coral branches have a matte look and are therefore dull. However, these can be polished to a beautiful shine for use as precious gems. A coral, therefore, like a pearl, can be categorized as an organic gem material.
The history tells us that the oldest coral to be used in coral jewelry was thousands of years ago around 3000 B.C. For almost 2000 years, the Romans used the coral gemstone extensively and made it popular.
In India, this gem is known by various names like mangal, moonga and pavlam. This gemstone is one of the Navagraha stones (stones that represent the nine planets that have a cosmic influence on all earthlings) and represents Mars (Mangal). It is related to attributes like courage, initiative, vitality, vigor, aggression. Coral has a special significance in Indian astrology and has found mention in ancient Indian Vedic Literature. It is one of the many starts that have an important place in the study of star signs.
Coral gemstones are known to have many healing powers and are specifically associated with the circulatory system of the body and known to have helped in curing many blood-related diseases. It is also used as a holistic aid in ensuring recovery from diseases indicated by the planet Mars, such as fever, cold and cough. For therapeutic purposes, this gem must be set in silver and worn in the ring finger.
Red coral jewelry is still worn as talismans in various cultures. It is presumed that the stone can help in protecting the wearer against evil spirits. In South India it is a tradition for married women to wear coral beads throughout their married life in order to bring peace and harmony into the marriage.
This stone is also said to avert accidents, discord and warfare. It is believed that wearing a coral can help in improving financial health and reducing debt.
The basic composition of this gem is calcium carbonate (CaCO3) in the form of fibrous calcite accompanied by around 3 percent magnesium carbonate (MgCO3), organic matter and traces of iron. It is a semi-translucent to opaque material. The specific gravity is 2.65 and refractive index ranges between 1.486 and 1.658. It`s hardness on the Mohs scale is 3.5 to 4.
There are different types of corals that provide corals of different hues. Gem coral occurs in white, pink, orange, red, blue, violet, gold and black. The black and golden coral types differ from the white to red in that they are not calcium carbonate, but rather a horny substance. But the ones that are mainly used in jewelry are red corals, pink corals and white corals.
Corals from Japan are said to have the ‘ox-blood’ red color. This specific color of the coral is valued highly and has a relatively higher demand in the gemstone market for jewelry like rings, earrings and necklaces. The pink to orange-pink variety are not found to possess a uniform color and therefore are not as popular as the red one. However, if the orange or orange-pink color is uniform, then pink coral jewelry, like a made from these stones has a fair amount of demand in the market.
The blue coral is found in shades that range from bright blue to grey blue. The black coral, sometimes, has brownish-yellow streaks on it.
Corals are usually cut into spherical and barrel shaped beads and cabochons. It is also carved into figures and statues. Thin branches of coral are usually polished, pierced and used unaltered in jewelry.
Coral roses (roses carved into corals) are very popular and are often used in gold jewelry too. The blue colour is not used in jewelry as often as the red since it does not look as good as a red coral. However, it is sometimes used in bead-necklaces. The black coral has a horny structure and its thin tentacles-like branches are used (just as they are naturally formed), in multi strand necklaces and bracelets.
They are not found in very large sizes. Sizes of up to 10cts are quite common, though.
It is mainly opaque and may have some surface markings that are clearly seen. Some of the most common surface markings on a coral are the wood grain effect, groove pits and striations.
Other than carat, one of the key drivers of its price is the richness of color. The price is lower for a gem that is lighter in color and the prices are higher for those that have a deep hue. Therefore light color coral fetches a lower price than deeper ones.
The red coral is the most expensive of all the varieties. It is almost as expensive as gold. A pink one, with no trace of orange and uniformity in color, costs almost as much as the red ones. If the color is uniform and bright red, then the price of a coral is likely to be around $5 per carat to $50 per carat depending upon the overall size. The demand for blue and black colour is not too high and therefore, they are not very valuable.
Various coral types and colors are found in different parts of the world.
- The best quality red sea corals are found in Italy around the Mediterranean Sea, Red Sea and the sea around Japan.
- Pink coral is found in a limited quantity in the Mediterranean region, Japan, China and India. The pink Hawaiian coral is also fairly famous.
- Blue coral is found in the coral reefs in the sea around Philippines.
- The black coral gem is gathered near the Hawaiian island, Australia, Red Sea, West coast of Africa and other parts of the Mediterranean region.
Though the reefs bearing coral is found in various locations, the Italians have a monopoly in the trade of this gem. Coral reef destruction can cause major issues with this industry and therefore its preservation is an important aspect that needs to be considered if the supply of this gem has to be unlimited and of good quality.
The most common treatments that are provided to a coral are dying, coral bleaching, coating and wax filling. Pale pink to white rough is dyed to produce the ox-blood (dark red) coral and angel skin coral (pink without the trace of orange). Black ones are bleached in hydrogen peroxide to give an attractive golden color. Epoxy filling and coating is done on the gem to remove cavities, cracks and other surface marks.
Imitation corals can be made by using simulants like paste (natural glass), plastic, wax, synthetic coral, conch pearl and marble calcite.
Gilson, a company that produces synthetic gemstones, produces synthetic corals with a wide range of colors from pale salmon pink to deep ox-blood red. The main distinguishing factor between fake, synthetic and natural coral is the distinct wood grain effect that the natural ones display. This effect is absent in its synthetic counterpart or fake coral. Fake red coral also tends to have an artificial richness in color.
Coral and pearl are biogenic precious stones. Although they are very different in appearance, they are chemically very similar. They both are comprised of calcium carbonate that has been deposited by various species of marine invertebrate animals and coelenterates like anthozoa.
Identification of Corals
When a drop of hydrochloric acid is put on a coral it shows a strong effervescence (bubbling of acid on the surface) due to the reaction that hydrochloric acid has with calcium carbonate. This is one method that is used to distinguish it from plastic, glass or other non-calcium carbonate material.
2) Gemmology by Peter G.Read
3) Handbook of Gem Identification - Richard T.Liddicoat, jr
4) Gems and Crystals - From the American Museum of Natural History - Anna S.Sofianides and George E.Harlow.
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