CORAL: South China Sea (red carving ~4 inch tall)
Coral is the external skeleton of a tiny, plantlike animal called the coral polyp. It lives in warm oceans in all tropical areas of the world. Although these creatures are only one millimeter in length, they grow as a colony on top of each other for generations. The resulting structures can be quite massive. Coral growths come in many shapes. The coral commonly used to make gems is branched and treelike. The largest sections of a coral's trunk are used for carvings. Most coral is cut into cabochons or made into a variety of shapes for use in necklaces.
Coral Value
Coral values are based on hue, saturation, size, cut, and polish. Top values for calcareous coral go to red, pink, and orange pieces. Other colors are graded separately. Highest values for conchiolin coral go to black, then brown. Gold color has additional value, especially if it shows a sheen. When polished, the color may shimmer through a transparent layer. Coral carvings can be quite valuable. The determining factors are the size and color of the piece as well as the skill of the artist.
"IMG_6338," koi fish carved from coral and rock, on display at the Taipei 101 tower, Taiwan, by Brian Jeffery Beggerly is licensed under CC By 2.0
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
Coral Information
| Data | Value |
| Name | Coral |
| Crystallography | Hexagonal (R). |
| Refractive Index | Calcite coral, 1.69 and 1.49, not usually measurable. Conchiolin (black coral) has RI of 1.56. |
| Colors | White, flesh pink, pale to deep rose red, salmon pink, red to dark red, blue (rarely), black. May be banded or zoned and show a cellular structure. |
| Luster | Dull to vitreous. |
| Polish Luster | Waxy, dull, vitreous |
| Fracture Luster | Dull |
| Fracture | Splintery, uneven |
| Hardness | 3.5-4 |
| Toughness | Good to fair |
| Density | 2.6-2.7. Note: Black coral, composed of conchiolin, is 1.34. |
| Birefringence | 0.160 |
| Cleavage | None |
| Heat Sensitivity | Sensitive |
| Luminescence | Pale violet or dull purplish red. |
| Spectral | Not diagnostic |
| Transparency | Semitransparent to opaque |
| Phenomena | Low surface sheen on gold coral |
| Formula | Calcareous coral, made primarily of calcite: CaCO3 Conchiolin coral, made primarily of conchiolin, an organic protein: C32H48N2O11 |
| Etymology | From the Greek korallion. |
“Coral Highness,” beadwork necklace with red corals, marble stones, and gold pendant, by Benita Nnachortam is licensed under CC By-SA 4.0
Comments
The gem use of coral began before recorded history. The ancient Greeks, Romans, and Native Americans used red, pink, and white corals extensively. Deep red, vivid pink, and clear white corals were highly prized. Inland cultures far from the sources of coral would trade for these resources.
“Object from Tibet 2,” tobacco pouch made from leather, silver, bronze, coral, and turquoises, by Vassil. Public Domain.
Although black and gold coral are relative newcomers to northern cultures, they have long been used as gem material in their native territories. They are found primarily off the coasts of Hawaii and Cameroon. Akori corals from Cameroon were highly prized before the eighteenth century. Hawaiian gold coral is the rarest gem coral variety and harder than other varieties. It was first described scientifically in the 1970s. The harvesting of this gem material, however, is currently restricted and cost prohibitive due to environmental considerations.
From the Georgian through the early Victorian Era, corals were very popular jewelry stones.
“Blood Coral Bead in Silver,” detail from an antique French rosary cross, by GorissenM is licensed under CC By-SA 2.0
The best red corals come from the Mediterranean. Most of the Mediterranean material is worked in Italy, but much is also sent to Hong Kong for cutting. Conversely, Italy is also a major buyer of Taiwanese material. Large quantities of white, pink, mottled, and oxblood corals from the South China Sea are cut in China and Taiwan.
“Sea Blossom Necklace – Pendant and Clasp Detail,” Bali silver, coral coins, and bronze pearls, by Christopher, Tania and Isabelle Luna is licensed under CC By 2.0
Although coral reefs are more commonly found in tropical areas, they can occur in colder, deeper water, such as those found to the west of Ireland.
Corals are becoming rare. Their harvest is restricted in most parts of the world. Be aware that any raw material you are offered could be illegal to possess.
Coral Varieties
There are two types of coral. Calcareous corals are composed primarily of calcite and come in whites, reds, and pinks. Conchiolin corals are composed of conchiolin, the same substance found in pearls and other shells. They come in black, brown, and gold colors. The conchiolin type is tougher and less brittle than the calcareous type.
A close relative of conchiolin corals is the rare blue coral. The hues are very nice, but the saturation is low, so these pieces tend towards gray shades.
“Image of Black Coral,” by Tsai ming lun is licensed under CC By-SA 3.0
Identifying Characteristics
- Calcareous: wavy, fibrous structure, cavities from polyps, high spot birefringence.
- Conchiolin: concentric, circular growth pattern (“tree rings”), white crescents in cross sections of branches.
Synthetics
In the 1970s, Pierre Gilson developed “created corals” to help protect the natural variety from destructive harvesting. This imitation red and pink coral has a specific gravity (SG) of 2.44. This is always lower than natural red and pink material. This synthetic has weak birefringence and lacks natural structure. Under high magnification, you can see a fine granular texture.
Plastic imitations can be identified through hot point testing or acid testing. These procedures should only be done by a professional.
Enhancements
Dyeing and bleaching are common treatments for coral.
Dyeing is used to improve or even changing color. The new color may fade. This process can be identified by magnification or a solvent test. The addition of dyes may produce phosphorescence.
Bleaching produces gold coral from black. This is a stable treatment. This process can be identified by magnification (which reveals a different texture) and lower SG and refractive index values.
Sources
Throughout the Mediterranean and Red Sea areas.
Australia; Cameroon; Hawaii; Ireland; Japan; Malaysia; Mauritius; South Africa; Spain; Taiwan
Stone Sizes
Branches may be several inches to several feet long but aren’t always thick.
“Koral1,” coral beads, by Kluka is licensed under CC By-SA 3.0
Coral Trade Names
Calcareous
- Akori, from Cameroon
- Algerian, low quality
- African Star, from South Africa, red, pink, violet and yellow/orange
- Angel skin, light pink
- Arciscuro, darkest red
- Bianco, white
- Carbonetto, darkest red
- Italian, good quality, white or pink
- Japanese, pink with white center
- Moro, high quality, light purplish red Japanese coral
- Tosa, average quality Japanese coral
- Ox blood, dark red
- Pelle d’angelo, light pink
- Rosa pallido, light pink
- Rosa vivo, medium pink
- Rosso, red
- Rosso scuro, dark red
- Salmon, light pink
- Sardinian, high quality, very hard
- Sicilian, low quality
Conchiolin
- Akori, blue
- Black, black to dark brown
- Blue, light to medium dark blue
- Gold, yellow to brownish yellow, may have sheen
- King’s, black
Care
Corals are sensitive to heat and should be cleaned only with a damp cloth and dried carefully. Mechanical cleaning methods, such as ultrasonic, are not recommended. Consult our jewelry cleaning guide for more information on cleaning jewelry pieces with sensitive gems.
The corals used to make jewelry were once the exoskeletons and homes of living creatures. “Coral polyps out at night” by Derek Keats is licensed under CC By 2.0
 | Destructive Gemstone Tests: Acid Testing |
