Its colour and rarity have long given coral the status of a precious stone. Recently, however, the gemological use of coral has been questioned due to its environmental impact and the fact that it is a marine animal. According to some, there is misinformation about this aspect.
Corals literally form trees on reefs, atolls and shallow-water coral banks.
Their numerous branches give them their tree-like appearance.
It is made up of small animals called polyps. They grow on the remains of the calcified skeletons of their dead mates in warm tropical waters.
Tree corals are found in a variety of colours depending on where they are found and the depth of the water.
According to Judith Crowe in her book “The Jeweller’s Directory of Gemstones”, there is a great deal of misinformation about the environmental damage caused by the collection of corals for gemstone use.
Of the 2,000 known species of coral, only a few are threatened with extinction and the most endangered are not used in the jewellery industry.
The height of corals ranges from 20 to 40 cm and the thickness of their branches can reach 6 cm.
It would be impossible to talk about coral without mentioning Torre del Greco, located at the foot of the Vesuvius volcano in the province of Naples, in the Italian region of Campania, the largest coral trading centre.
The largest living coral reef is the Great Barrier Reef off the coast of Queensland, Australia. It consists of 2,900 reefs and 300 atolls (circular reefs).
It is home to around 360 species of coral and a rich biodiversity of fish, molluscs, crustaceans, starfish, etc.
Synthetic coral and imitations
Because coral is a rare gem, it is often imitated using various products and substances. It’s important to distinguish between the aspects of imitating and synthesising a gem.
In the case of synthetic gemstones, scientists and technicians attempt to replicate the structure of the material being synthesised.
A synthetic gemstone therefore has all the characteristics of a natural gemstone, but is produced in a laboratory.
Imitations are not concerned with the structure of the material, but only with the visual aspect.
Imitations are made from a variety of materials including porcelain, glass, plastics and resins.
Synthetic gemstones are well accepted in the global jewellery market.
Firstly because they retain the same characteristics as the original gem and secondly because they are cheaper.
The rarity of many types of gemstones and their high cost has led to a growing market for synthetic gemstones.
Synthetic coral can be carved, cut, polished and engraved, which is difficult with natural coral, which has low hardness and is fragile.
To check if a piece of coral is an imitation, simply expose it to cold, dilute hydrochloric acid. Imitations don’t react to acid, while natural coral will produce a very visible effervescence when exposed to this acid.
Gemological characteristics of coral
- Low hardness – 3-4 Mohs.
- Relative density – 2.60-2.70.
- Fracture – irregular, fractured and brittle.
- Crystal system – trigonal, microcrystalline.
- Transparency – translucent, opaque.
- It has a slightly cylindrical, hollow shape.
- Its composition is 87% calcium carbonate, 7% magnesium carbonate and other substances.
Corals live in association, in true symbiosis, with unicellular algae such as zooxanthellae.
This gives corals the ability to grow very quickly and it is these organisms that give corals their vibrant colours.
Corals turn whitish when environmental conditions change. They lose some or all of their algae in unfavourable environmental changes, losing their vibrant colour.
They are pink, red, orange and white.
Coral bleaching has occurred on a large scale in recent decades, and depending on the length of the environmental imbalance, the colony may die in whole or in part.
Excessive lighting and prolonged exposure to ultraviolet light can also cause bleaching.
Rising global temperatures are also affecting coral life.
In jewellery they are used as gems in many different types of jewellery and also as cameos. They are also used in jewellery in their raw state.
There are also black and blue corals, which are less valuable.
Types of coral
1. Sponge Coral
Sponge coral is found on reefs. It is not a rare or precious type of coral. It has a very low risk of extinction.
In its natural state it is found in red, purple and yellow colours.
However, it is common to find it dyed and with the application of resins to obtain a lustre.
2. Horse Coral (Corallium rubrum)
Noble coral or Corallium rubrum is the most sought after by jewellery designers and consumers in general.
It lives in the world’s most biodiverse ecosystem and is considered one of the most endangered.
It takes a lot of work to turn raw coral into a gemstone, which is usually very expensive.
This process involves cutting, carving and polishing. They are fragile.
3. Bamboo coral
Bamboo coral is so called because the growth of its branches resembles bamboo.
In its raw state it is very delicate and difficult to cut.
4. Secundum coral
They are pink and white and come from the Pacific Ocean.
5. Japanese coral
They are red and dark red, from Japan.
6. Konojoi Coral
They are white and sometimes have pink spots. They come from Japan and the Philippines.
7. Elatius Coral
Orange, from Japan and the Philippines.
Caring for coral
Like all organic gemstones, coral is not very resilient and requires care, both in the making of the jewellery and in its use by the consumer.
Coral should be kept away from intense heat. They should not be exposed to aggressive chemical products.
It is not advisable to scrub them with hard brushes as even treated coral is still fragile.
It is important to avoid contact with metal parts that could scratch it. They should be cleaned with a neutral detergent.
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