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  • Coral

    fluorescent coralFluorescent coral in Underwater Observatory Marine Park, Eilat.
    Photo credit: 
    Tiia Monto

    Coral is the term used to describe a vast group of marine invertebrates (animals without backbones) that collectively belong to the Phylum Cnidaria of the Class Anthozoa.  While many people know of the red coral used in jewellery making, they would be surprised to learn that the term actually applies to at least 100,000 known species found throughout the world's seas, from the polar regions to equatorial reefs and at all depths from the intertidal zone to the bottoms of the deepest oceanic trenches - and scientists estimate that there may be another 900,000 species awaiting discovery.  Many may also be surprised to learn that corals are animals, not plants.  The tiny individuals, known as polyps, continuously excrete the calcium carbonate scaffolding that houses them, forming communal colonies that may be small and delicate or huge barrier reefs.  Corals also occur naturally in just about every color imaginable, from shades of white, orange, pink, red, tan, gray, lavender and blue, all the way through to black.

    Corals may be subdivided into two broad subclasses. The first is Alcyonaria, containing the "soft" corals, which means that their skeletons are always internal.  These include the bamboo corals and the so called "gorgonian" corals, comprising sea pens, sea fans, gold corals, most pink to red corals, blue coral and organ pipe coral. The second subclass Zoantrharia contains the sea anemones, the stony reef-building corals that form massive barrier reefs and the solitary black, thorny corals. Corals play a vital role in helping to maintain the sea's delicate ecological balance and provide food and shelter for countless millions of other organisms.  As a renewable human resource, corals are the slowest growing organisms of any known fishery past or present.

    History of Use

    coral earrings
    Coral rosebud carved earrings. Photo credit: Kashgar


    Today coral is classified along with pearls, ivory and amber as an "organic" gem, however it is only relatively recently that its animal nature has been known.  In ancient times it was considered to be simultaneously animal, vegetable and mineral, and until the 1700's naturalists believed coral to be a marine plant that petrified when removed from water.

    Coral has been valued and used for jewellery fabrication and decoration since the earliest history of man.  It was used as a form of currency along with amber by Paleolithic man and pieces of Mediterranean coral have been found in the Neolithic city of Catal Huyuk, Central Anatolia that date to 8,000 years BCE. In ancient Egypt coral jewels were popular during the times of Ptolemy, over 2000 years ago.  Coral was traded across the ancient and modern world via established trade routes to Italy, where the major workshops were located, and then to Central Europe, Siberia and the Celtic countries, while other types of coral traveled from the Indian Ocean across Asia to Tibet and Mongolia. The Victorians adored coral, carving it into delicate roses and cameos, and since the late 1800's American Indians have incorporated red coral into many of their jewellery designs to take into account the tastes of settlers and tourists. Today coral is equally prized in jewellery making, while live corals and products such as coral rock have become increasingly popular in the marine aquarium trade.

    Since ancient times coral was believed to cure madness, give wisdom, prevent sterility, calm storms, promote positive forms of social activity and provide protection against plague and pestilence.  It was also thought to enable travellers to cross rivers safely. Red coral symbolises attachment and devotion.  Amongst tribal cultures past and present it is universally believed to ward off the evil eye and bad luck, and it is still common for children and young women to wear coral necklaces (or even ceramic coral imitations, called "mountain coral" in the trade) for protection. In Vedic astrology red coral is associated with the planet Mars.

    Coral Usage Today

    Although as many as 1 million species of coral exist today, only 40 or so are subject to commercial usage.  These may be summarised as follows:

    Red - Pink - White or Precious Coral

    red coralThe so-called "precious" or red - pink - white corals belong to the Genus Corallium.  Twenty-five species of Corallium have been named, of which six species are commonly traded, including the famous Mediterranean red coral (Corallium rubrum), the Pacific Ocean species Hawaiian pink coral (C secundum), red coral (C regale), pink coral (C laauense), and the Midway deep sea coral (Corallium sp nov).

    Coralliums are very slow growing - Mediterranean red coral at rates of less than 1mm per year and the Pacific Ocean species between 3 and 8mm per year.  They are also relatively long lived, attaining ages of up to 75 years old, and heights of up to 1 metre.  Mediterranean red coral is found in depths of 10 to 280 metres, often in caves and crevices, while the red and pink corals of the west Pacific are found at depths of 350 -1500 m and prefer areas with moderate to strong water currents.

    Mediterranean red coral (which can itself range in colour from pale orange to the deepest of oxblood reds) had been used for thousands of years in the production of jewellery, religious objects and medicine, and remains the most highly valued of all the corals. Italy is the main producer of red coral objects and still holds large stockpiles harvested prior to the 1980's. Pink coral, also considered very desirable by the jewellery industry, is very dense and relatively hard. It can span the entire spectrum of pinks, from almost white to nearly red. While all shades of pink coral are coveted, pale pink Angelskin coral is preferred by the jewellery industry.

    Because of their slow growth and value to the jewellery industry, the red-pink-white corals are vulnerable to over-exploitation and many populations in both the Mediterranean and the Pacific have been over-harvested; yields from the Mediterranean have been reduced by 66 percent in the last 25 years. In the past these corals were harvested using non-selective dredging equipment that caused widespread damage, not only to the coral colonies but also to entire undersea habitats.  Dredging has now been banned throughout the Mediterranean Sea and around Hawaii and harvesting is carried out using more selective methods such as scuba diving or submersible collection.  Somewhat surprisingly, the Coralliums are not listed in either Appendix I or II of the Convention on International Trade in Endangered Species (CITES), although their trade is controlled via other international fishery management Directives and they are often subject to strict domestic legislation.

    red coral
    Red coral branch of the species Corallium rubrum from Italy (height - 4.9 cm) ready for jewellery making. Note that the colour is natural and not added with dyes. Photo credit: Jeffrey A. Scovil.

    Precious corals, irrespective of their colour or origin, may be identified in hand specimens by their solid dense texture, few holes being visible in its polished external surface other than clusters of pin point holes, and the presence of parallel longitudinal striations on the surface of the coral.   Close examination should also distinguish between natural and reconstituted coral, which is made from coral fragments that have been pulverised into a powder, soaked in binding agents, then pressed into a solid mass to be re-cut and dyed. Red corals may also be imitated in tribal cultures with ceramic materials, but carry none of the imperfections and striations associated with natural corals.

    Black coral

    black coralBlack coral in situ (unidentified Scleractinia). Photo credit: Aaron

    The black corals belong to 4 separate families, the Antipathidae, Leiopathidae, Schizopathidae and Myriopathidae containing 150 species so far identified, of which 8 are subject to international trade.  Black corals are covered in spines and may be either branching or wire-like without branches and when alive are white, yellow, orange, red or green in colour.  Colonies may reach up to 1.8 m in height and are distributed worldwide.  Small fisheries for black coral exist in all oceans but are most developed in the Indo-Pacific and Caribbean regions.  

    Black coral colonies are slow growing and long-lived.  They are found at a wide range of depths, from 1 - 6000 metres, but are generally distributed between 30 and 80 metres. They tend to be associated with strong currents and low light conditions, either by growing in deep water or in turbid areas of shallow water.

    Because of their unusual  and lightweight nature, black corals are highly valued and have been used for centuries in jewellery making and as medicine.  An aquarium trade in live specimens has also been reported.  Harvesting is mainly carried out by divers and by submersibles, although net dredging has been used in the past. Black corals are slow to recover from harvesting and are currently considered to be globally threatened.  Some black corals are listed in CITES Appendix II, which means that a CITES export permit is required to bring anything made from black coral into the 158 countries that have ratified the Convention (this includes the UK, other countries in the European Community, the USA, Canada, and Australia). In addition to CITES the export of black coral is banned or restricted by a number of countries.  However it should be noted that harvesting in some locations such as Hawaii is carefully and responsibly managed to ensure that collection is sustainable.

    black coral
    Unpolished rough black coral sticks. Photo credit: Peter Rohm,
    Rohm GmbH & Co Kg, Linz, Austria.

    Black corals may be identified by their tree-like rings in cross-section, the appearance of spiny stub-ends in polished material and a 'salty burnt hair' aroma when a hot point is applied to the surface.  Black corals are also far lighter than other corals and have a red-brown appearance under strong light.


    Bamboo Coral

    bamboo coral

    Bamboo coral (Keratoisis sp.), discovered at 1,455 m depth at Davidson Seamount. Photo credit: NOAA/MBARI 2002.

    Bamboo corals belong to the Family Isidiae of which it is estimated there are some 38 genera and 138 species worldwide - estimated because new species are still being described on a regular basis.  In spite of the fact that bamboo corals have now largely taken the place of the red corals in jewellery manufacture, very little is actually known about them.  Currently four species across four genera are harvested for commercial purposes.

    Bamboo corals get their name from the distinctive appearance of their jointed skeletons, with long white nodes separated by striking black internodes, just like their land based namesakes.  They live mainly on the deep-ocean floor at depths greater than 1 km and while they are found in oceans all over the world, the greatest concentrations are distributed around New Zealand and some other areas of the Pacific such as the Philippines.  All coral species are long-lived, but it is thought that some species of bamboo coral can live for centuries, possibly even thousands of years (that is, colonies, not individual polyps).  They may also grow several metres high, forming dense thickets that provide vital habitat for small invertebrates and nurseries for many commercial fish species.  Because they are so long lived and also form growth rings just like trees, bamboo corals may be ideal for studying climate changes across centuries.

    Natural bamboo coral is a creamy white to grey colour with a discernible dark core, however it readily takes dye and is often coloured pink or red in imitation of Mediterranean coral. Most of the new and legal coral coming onto the market today is likely bamboo coral from the Philippines and China.

    bamboo coral
    A bamboo coral skeleton (Ceratoisis flexibilis) with living tissue removed. 
    Photo credit: NOAA

    When whole, bamboo coral is easy to distinguish from other corals because of its bamboo-like appearance.  In cross section it typically has tree like rings and a dark rough core.  When colour enhanced and cut into beads it can be difficult to distinguish from the precious corals.  Besides basic dipping, the coral may be injected with dye to saturate it all the way through.  It may be possible to distinguish dyed bamboo from precious coral beads by the evenness of colour, ie a lack of striation or colour variation of any sort within or between beads - for this the use of a 10x eyeglass is recommended.

    bamboo coral jewellery - a bbracelet
    Dyed bamboo coral. Note the lack of striation and large black inclusions.
    Photo credit: Kashgar.

    It should be emphasised that bamboo corals are a legal and much less expensive alternative to the precious corals.  They now satisfy much of the public demand for coral products, which in turn is allowing the recovery of the Mediterranean and Pacific colonies and their associated eco habitats.  This is true so long as the coral is correctly identified to the buyer as bamboo and not as one of the precious corals.

    Sponge Corals

    fan coral
    Knotted fan coral (Melithaea ochracea), Pulau Tioman, West Malaysia.
    Photo credit: Anders Poulsen, Deep Bluehttp://www.colours.dk

    The so-called sponge or soft corals are neither particularly soft nor spongy.  Their name comes from the very open skeleton that they secrete, full of holes and irregular surfaces.  They are primarily found in shallow, tropical and sub tropical waters and are relatively short-lived compared to their deeper water cousins. There are many hundreds of species which occur in a variety of natural colours, but only two types are utilised in the jewellery industry, the red and blue sponge corals (a much larger number of species are popular with the aquarium trade).

    Red Spongy Coral (Melithaea ochracea) is relatively new to the jewellery industry, but is fast growing in popularity because of its availability and relatively low price.  Fan shaped and naturally red in colour with yellow-orange-brown streaks, it must be stabilized with resins because of its highly porous nature. Poorer quality material may be dyed, and it is reported that China and Indonesia may also be pressing fragmented material.  Red spongy coral is harvested in the South China Sea, from Taiwan to Indonesia, and is now apparently being farmed in some of these areas.  Red spongy coral is not an endangered species, is not listed with CITES and is considered worldwide to be a sustainable resource.

    spongy coral
    Heliopora coerulea, Samoa, America, 2014 

    Blue Spongy Coral (Heliopora coerulea) can be tree-like, plate-like or column-like and may reach several metres in height. It is considered to be an important reef-building coral in the Indo-Pacific oceans, with the largest colonies being found around Japan and Hawaii.  Its blue colour is caused by the deposition of iron salts in its calcium carbonate skeleton.  While blue spongy coral has some popularity in the jewellery industry (it requires stabilisation just like it's red cousin), it is mainly harvested for the live aquarium and marine curio trades. However, the species is considered threatened and is listed in CITES Appendix II.  In addition several countries have legislation banning or limiting the collection or trade of blue coral.

    In jewellery, spongy corals may be easily identified by their very porous, variegated and streaky surface.

    spongyform red coral
    Spongyform red coral polished and ready for jewellery making.
    Photo credit: Kashgar

    Gold Corals

    gold coral
    Geradia sp. (gold) coral Photo credit: NOAA Hawaiian Undersea Research Lab

    Of all the corals, the golds are probably the least well studied. There are only four known species that occur within three Families; Narella sp, Gerardia sp, Callogorgia gilberti and Calyptrophora sp.

    These corals are particularly unusual in that they secrete a golden keratin protein but no calcium carbonate.   They are also considered "parasitic" in that they settle as free-swimming larvae onto the skeletons of other corals, eventually overgrowing and smothering them.  They mostly occur in deep waters, are associated with the bamboo coral colonies they like to colonise and may be the slowest-growing of all the coral species, with colonies reaching thousands of years in age.   Unlike the bamboo corals, the golds may only lay down their growth rings once a decade.

    Gerardia sp. is legally and commercially harvested from Hawaiian waters, and this is the primary source of the small amount of gold coral jewellery that finds its way onto the market.  While the gold corals are not listed in either of the CITES Appendices, their limited distribution, extremely slow growth rates and poorly understood biology suggest that a market for gold corals should not be developed or encouraged.

    Gold coral may be identified by its tree like growth rings in cross section, it's even gold colour and dimpled surface, and a 'burnt hair' aroma when a hot point is applied to it's surface.

    More recently, Lawrence Livermore National Laboratory scientists have used the skeleton of Geradia sp. coral as a strip-chart recorder for what the coral has fed upon. Because these deep sea corals are some of the oldest living marine organisms known to man, they serve as a useful record keeper for ocean and climate changes.


    Care and Maintenance of Coral in Jewellery

    Because of its relative softness (3 ½ to 4 on Mohr's scale), coral can scratch and abrade easily.  Bright lights may cause it to darken while excessive heat can damage the finish.  Acids (even those found in sweat) or solvents such as the chlorine in swimming pools, alcohol, nail polish remover and make up remover will soften coral over time.  For these reasons, coral jewellery should be removed when washing hands or remving makeup, should not be worn in the shower and should never be left on window sills or in the sun.  Coral is best stored in a cloth bag in an environment with an even temperature and humidity.

    Updated 28th March 2017

    References and Further Reading:

    Brown, G. (1976) Two new precious corals from Hawaii. Australian Gemologist. 12, 371-377.

    Brown, G. (1988) Bamboo coral: A new precious coral from Hawaii. Australian Gemologist. 16, 449-454.

    Dietrich, RV (2015) Corals. http://stoneplus.cst.cmich.edu/zoogems/coral.html

    Etnoyer P (2002, revised 2008) What Species is a Bamboo Coral?www.oceanexplorer.noaa.gov http://oceanexplorer.noaa.gov/explorations/02alaska/logs/jul14/jul14.html

    Grigg, R. & Brown, G. (1991) Tasmanian gem corals. Australian Gemmologist. 17, 399-404. 

    Lawrence Livermore National Laboratory (2015) Scientists discover shifts in climate-sensitive plankton over the past millennium. Accessed 28th March 2016.


    Staff Report (March 5th 2009) Discovery - 7 New Species Of Bamboo Coral Near Hawaii.  Scientific Blogging Science 2.0http://www.scientificblogging.com/news_releases/discovery_7_new_species_bamboo_coral_near_hawaii

    Sánchez J, Tracey D, Neil H and Marriott P (2004) Coral Rings in the Deep Ocean: using SEM to date New Zealand's bamboo corals  NIWA Science Vol.12 No.4http://www.niwa.cri.nz/pubs/wa/ma/12-4/coral

    Schultz N (March 2009) Coral colony as Old as the Pyramids New Scientist Online News http://www.newscientist.com/article/dn16825-coral-colony-as-old-as-the-pyramids.html?DCMP=OTC-rss&nsref=online-news

    Wikipedia.  Coral. Referenced 16 April 2009

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