What is Coral?
Precious coral or red coral is the common name given to Corallium rubrum and several related species of marine coral. The distinguishing characteristic of precious corals is their durable and intensely coloured red or pink skeleton, which is used for making jewellery.
Coral is not a mined stone or mineral, but an organic gemstone. It is the hardened, skeleton-shaped result of secretions continuously deposited by marine polyps. It looks like a colourful underwater plant, but it is actually a priceless gift from the sea.
Habitat
Coral is found in clear at between 15 and 160 feet. The greater the depth, the lighter the coral's colour. Coral reefs are found in almost all of the world's oceans. The biggest is the Great Barrier Reef, off the coast of Queensland, Australia.
Italian red coral is in the highest demand In the jewellery industry and is considered the finest quality. The finest specimens come from Sardinia (Italy) and Tunisia. Coral from Japan and China is considered slightly inferior in quality, but it is more expensive than Italian red, because it is less abundant. Coral is also found off the coast of Washington State (U.S.A) and the Aleutian Islands off of Alaska.
Coral Gemstone
Unlike most other gemstones which are of mineral origin, Coral is organic, formed by living organisms. It forms from branching, antler-like structures created from coral polyps in tropical and subtropical ocean waters. When the coral polyps die, the hardened skeleton remains, and this material is what is used as a gemstone. Most coral is white, but nature can create coral in several other colours, including the popular orange to red forms. This Red Coral, or Precious Coral as it is often known by, is the most used gemstone form of Coral. In fact, the colour known as coral is derived from the typical pinkish-orange colour of many red Coral gemstones.
The hard skeleton of red coral branches is naturally matte, but can be polished to a glassy shine. It exhibits a range of warm reddish pink colours from pale pink to deep red; the word coral is also used to name such colours. Owing to its intense and permanent coloration and glossiness, precious coral skeletons have been harvested since antiquity for decorative use. Coral jewellery has been found in ancient Egyptian and prehistoric European burials, and continues to be made to the present day. It was especially popular during the Victorian age.
Precious coral has relative density of 3.86 and hardness 3.5 on the Mohs scale. Due to its softness and opacity, coral is usually cut en cabochon, or used to make beads.
Coral is typically worn in pieces that weigh at least three carats. A ring or pendant can hold a piece weighing up to twelve carats. Red coral may be set in gold or silver.
Properties of Coral Gemstone
| Chemical Formula | Mostly calcium carbonate |
| Colour | White, Red, Orange, Pink, Gray, Black |
| Hardness | 3 - 4 |
| Crystal System | Amorphous |
| Refractive Index | 1.48 - 1.65 |
| SG | 2.6 - 2.7 |
| Transparency | Translucent to opaque |
| Double Refraction | -.172 |
| Luster | Vitreous, waxy |
| Cleavage | None |