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Red coral is a marine organism, not a true rock or mineral in the strict geological sense, but rather a biogenic mineraloid. It is the skeletal remains of colonies of marine polyps, primarily Corallium rubrum, found in deep-water environments. Its characteristic color ranges from pale pink to deep red, often with a vitreous to waxy luster when polished. It forms branching, tree-like structures that are dense and relatively hard. The internal structure is typically compact, sometimes showing concentric growth rings or a fine granular texture. It is susceptible to acids due to its calcium carbonate composition.
How to Identify
- Color
- Ranges from pale pink (angel skin coral) to deep ox-blood red. Can also be orange-red.
- Luster
- Dull to waxy when unpolished; vitreous (glassy) when polished.
- Texture
- Smooth when polished; naturally exhibits a fine granular or fibrous texture. May show growth rings or a porous surface in unpolished specimens.
- Crystal Form
- Amorphous to cryptocrystalline (calcite microcrystals). Forms branching, tree-like structures.
- Cleavage
- None (amorphous/cryptocrystalline structure).
- Geological Environment
- Deep-water marine environments, typically on rocky seabeds or submerged caves, at depths ranging from 10 to 300 meters (33 to 984 feet). Requires stable temperatures, moderate currents, and low light conditions.
Key Facts
- Hardness: 3 to 4 on the Mohs scale (relatively soft)
- Specific Gravity: 2.6 to 2.7
- Crystal System: Amorphous to cryptocrystalline (composed of microcrystalline calcite)
- Color: Various shades of red, pink, and orange-red
- Luster: Waxy to vitreous
- Transparency: Opaque
- Fracture: Splintery to conchoidal
- Cleavage: None
- Composition: Primarily calcium carbonate (CaCO3) in the form of calcite, with organic binders.
Quick Check
- Color: Pink to deep red
- Luster: Waxy to vitreous (polished)
- Streak: White
Physical Characteristics
- Crystal Habit: Branching, arborescent (tree-like) structures, often massive or columnar.
- Cleavage Type: Absent
- Fracture Type: Splintery to conchoidal, uneven.
- Tenacity: Brittle
- Luster Type: Waxy to vitreous (when polished)
Formation
Red coral is formed by colonies of marine polyps (Corallium rubrum and related species) that secrete a hard, calcareous exoskeleton. This exoskeleton is composed primarily of calcium carbonate in the form of calcite, reinforced by organic compounds. The polyps extract calcium and carbonate ions from seawater to build their skeletal structure. Over time, as polyps grow and die, their skeletons accumulate, forming the branching, tree-like structures characteristic of red coral. These structures grow very slowly, typically a few millimeters per year.
Usage
Historically and currently used extensively in jewelry (beads, cabochons, carvings), decorative objects, and traditional medicine in some cultures. Its vibrant color and relative hardness make it a desirable material for ornamental purposes. Due to its slow growth and overharvesting, many species of red coral are now protected, and its trade is regulated.
Age Distribution
Modern (Holocene) to Pliocene, with fossil records extending further back for related coral forms.
Where to Find
Mediterranean Sea
Historically the primary source, particularly off the coasts of Italy (Sardinia), Spain, France, Algeria, and Tunisia. Corallium rubrum is endemic to this region.
Western Pacific Ocean
Other Corallium species (e.g., Corallium japonicum, Corallium elatius) are found off the coasts of Japan, Taiwan, and the Philippines, often at greater depths.
Atlantic Ocean
Some species are found off the coast of the Canary Islands and Cape Verde.
Finding Tips
Ethical Sourcing
Due to overharvesting and conservation concerns, it is crucial to ensure any red coral purchased or collected is ethically and legally sourced. Many species are CITES-listed (Convention on International Trade in Endangered Species of Wild Fauna and Flora).
Distinguishing from Imitations
Genuine red coral will feel cool to the touch and have a specific gravity of 2.6-2.7. Plastic imitations will be lighter and warmer. Dyed materials like howlite or marble will often show uneven color distribution or a different internal structure. A drop of dilute hydrochloric acid will cause genuine coral to effervesce (fizz) due to its calcium carbonate content, but this test is destructive and should be avoided on finished pieces.
Environmental Awareness
Direct collection of red coral from the ocean is highly regulated or prohibited in most areas due to its protected status and the fragility of coral ecosystems. Collectors should focus on antique pieces or legally sourced material from reputable dealers.
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Plastic Imitation Coral
N/A
Also known as: Faux Coral
Dyed Howlite
Calcium borosilicate hydroxide
Also known as: Turquenite (when dyed blue)
Scientific Classification
- Mineral Class
- Biogenic Mineraloid (not a true mineral)
- Group
- Carbonates (specifically, calcium carbonate)
- Crystal System
- Amorphous to cryptocrystalline (composed of microcrystalline calcite, which is trigonal)
- Chemical Formula
- CaCO3 (with organic impurities)
- Composition
- Calcium carbonate (calcite) with a small percentage of organic material (conchiolin).
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