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Organic Gems: Red Coral

Red Coral & Bamboo Coral Jewelry

Red Coral

Source: Mediterranean Sea, Sea of Japan

Corals are major contributors to the physical structure of coral reefs that develop in tropical and subtropical waters. Due to its unique and striking color, red coral has become a popular (macrobiotic) gemstone for jewelers in recent years. Pure red coral (Corallium rubrum), also known as 'fire coral' or 'ox blood coral' is becoming very rare due to the high demand for perfect specimens used in jewelry making, and the stress that over-collection is placing on the environmental health of coral reefs.

Fire corals are deep-dwelling corals that are typically found at depths or 50 m to over 200 m. A single coral "head" can be the home to thousands of individual polyps, each polyp only a few millimeters in size. A colony of polyps function as a single organism, sharing nutrients via an interconnected gastrovascular system. All of the polyps in the colony are clones of each other, sharing the same genetic code. Each new polyp generation lives on the main calcareous skeletal (exoskeleton) that is the remains of previous generations.

Red Coral Jewelry

The coral formation process creates a structure that is unique to the particular coral species, but the ultimate physical characteristics of each coral species is also subject to environmental influences. It is believed that corals first appeared during the Cambrian period, around 570 million years ago, yet Cambrian corals are extremely rare as fossils. Fossilized Rugose and Tabulate corals from the Ordovician period around 488 million years ago, are fare more prevalent.

Conchiolin & Calcareous Coral

There are two distinct types of coral: Calcareous Coral which is primarily composed of calcite, and Conchiolin Coral which is primarily composed of protein. Formation of a calciferous exoskeleton involves deposition of calcium carbonate by the polyps from calcium ions that are isolated from the surrounding seawater. Conchiolin forming proteins are made up of organic macromolecules consisting of polysaccharides and proteins bound together with aragonite crystals.

Precious red coral, fire coral, or ox blood coral that is used in jewelry is a semi-translucent to opaque material that is frequently dyed to enhance color. Corals can also be impregnated with resins or epoxies to fill surface fissures and flaws. Black and Red Corals used in jewelry are Calcareous Corals that are much softer than other gems. Gem-grade black coral, which typically came from Mexico and the Caribbean, is no longer commercially available [2].

Simulated & Reconstituted Red Coral

Reconstituted coral is made from natural solid material, or coral fragments that have been pulverized into a powder, soaked in binding agents, then pressed into a solid mass to be re-cut. The reconstituting process is typically accompanied by dying to enhance or create a vivid red color.

Red Coral, Fire Coral & Ox Blood Coral Properties

Crystal System organic
Mohs Hardness Scale 3.5 to 4.0
Refractive Index 1.48
Specific gravity (SG) 2.65 to 2.68
Surface Luster waxy to vitreous
Toughness fair
Fracture conchoidal
Gem Color black, pink, red
Chemical Composition CaCO3 calcium carbonate, aragonite

Bamboo Coral

Another interesting species of coral that is used in jewelry is bamboo coral, Keratoisis profunda, also known as "sea bamboo," which has a unique banding pattern of greens and reddish-browns. Like red fire coral, bamboo coral are dyed to enhance color and their natural color is typically gray. bamboo coral can also be dyed either pink or red.

Bamboo Coral Habitat

Bamboo coral habitat (photo: noaa.gov)

   Polished Bamboo Coral Beads

Polished & dyed bamboo coral beads

Bamboo corals are in the Family Isididae, which are "octo-corals," having eight pinnate tentacles on each polyp that can be either retractile, or non-retractile. Very little is known about bamboo corals, but scientists think that they can live for centuries. Bamboo corals occur in waters all over the world.

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Bibliography on Red Coral, Ox Blood or Fire Coral & Bamboo Coral

1. Paul R. Shaffer, Rocks, Gems and Minerals . Martin's Press

2. M. Prost, In the Red: Red & Pink Coral . www.colored-stone.com

3. S. Rossi, Survey of Deep-Dwelling Red Coral . Marine Biology

4. Peter Etnoyer, What Species is a "Bamboo Coral"? . oceanexplorer.noaa.gov

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