Its colour and rarity have long given coral the status of a precious stone. Recently, however, the gemological use of coral has begun to be questioned on the grounds of environmental damage and the fact that it is a marine animal. According to some opinions there is misinformation on this aspect.
Literally tree-shaped, corals form on reefs, atolls and shallow-water coral banks.
Their numerous branches give them their tree-like appearance.
It is made up of tiny animals called polyps. They grow on the remains of calcified skeletons of their dead mates in warm tropical waters.
Tree corals are found in distinct colours depending on where they originate and the depth of the water.
According to Judith Crowe, in her book “The Jeweller’s Directory of Gemstones”, there is a high degree of misinformation about the environmental damage caused by collecting corals for gemological use.
Of the 2,000 known types of coral, only certain varieties are at risk of 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 reaches 6 cm.
It would be impossible to talk about coral without mentioning Torre del Greco, at the foot of the Vesuvius volcano in the province of Naples in Italy’s Campania region, the largest centre for coral trade.
The largest living coral reef is found in the Great Barrier Reef off the coast of Queensland in 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
Being a scarce gemstone, coral is often imitated using different products and substances. It is important to differentiate the aspects of imitation and synthesising a gem.
In the case of synthetic gemstones, scientists and technicians seek to iel reproduction of the structure of the material to be synthesised.
Thus, the synthetic gem has all the characteristics of the natural gem, however, it is made in a laboratory.
In the case of imitations, there is no concern with the structure of the material, but only with the visual aspect.
Imitations are made with different types of materials such as porcelain, glass, plastics and resins.
Synthetic gems are well accepted in the world jewellery market.
Firstly because they retain the same characteristics as the original gemstone, and secondly because they are cheaper.
The rarity of many gem types and their high cost has increasingly expanded the market for synthetic gems.
Synthetic coral can be carved, cut, polished and engraved, which is difficult with natural coral which has low hardness and is fragile.
To check whether a coral is an imitation, it is sufficient to subject it to cold dilute hydrochloric acid. Imitation coral does not react to the acid, while natural coral has a very visible effervescence when subjected to this acid.
Gemological characteristics of corals
- They have low hardness – 3-4 Mohs.
- Relative density – 2.60-2.70.
- Fracture – irregular, shattered and brittle.
- Crystalline system – trigonal, microcrystalline.
- Transparency – translucent, opaque.
- It has a slightly cylindrical and hollow shape.
- Their 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 the coral the ability to grow very quickly and it is these organisms that give the coral its vivid colour.
Coral turns whitish when environmental conditions change. With unfavourable environmental changes they shed their algae partially or completely, thus losing their vibrant colour.
They are found in pink, red, orange and white colours.
Coral bleaching has occurred on a large scale in recent decades, depending on the time of environmental imbalance may be the total or partial death of the colony.
Excessive lighting and prolonged exposure to ultraviolet light can also cause bleaching.
The rise in the planet’s temperature also affects the life of corals.
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 the less valuable black and blue corals.
Species of corals
1. Sponge coral
Sponge coral is found on reefs. It is not a rare or valuable type of coral. It has a very low risk of extinction.
In its natural state it is found in red, purple and yellow hues.
However, it is common to find it dyed and with the application of resins to obtain lustre.
2. Noble coral (Corallium rubrum)
The noble coral or Corallium rubrum is the most appreciated by jewellery designers and consumers in general.
It lives in the most biodiverse ecosystem in the world and is considered one of the most vulnerable.
It is very labour-intensive to convert raw coral into gemological element, which usually gives it high cost.
This process is done by cutting, carving and polishing. They are fragile.
3. Bamboo coral
Bamboo coral was named so because the growth of its branches resembles bamboo.
In its raw state it is very sensitive and difficult to work (lapidary).
4. Coral secundum
They are pink and white in colour from the Pacific.
5. Japonicum coral
They are red and dark red in colour, coming from Japan.
6. Konojoi coral
They are white and sometimes with pink spots, coming from Japan and the Philippines.
7. Elatius coral
They are orange, from Japan and the Philippines.
Care for corals
Like any organic gemstone, coral is not very resistant and requires care, both in the production of the jewellery and in its use by the consumer.
Corals should be kept away from strong heat.
They should not come into contact with aggressive chemicals.It is not advisable to rub it with hard brushes, as even treated coral is still fragile.
It is important to avoid contact with metal parts that can scratch it. For cleaning they should be washed with neutral detergent.