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Coral is a biogenic sedimentary rock composed primarily of calcium carbonate, secreted by colonial marine polyps. It typically exhibits a porous, skeletal structure reflecting the growth patterns of the coral colonies. The color can vary widely depending on the species of coral, the presence of symbiotic algae (zooxanthellae), and mineral impurities. It is a significant component of coral reefs, which are among the most biodiverse ecosystems on Earth.
How to Identify
- Color
- Highly variable; white, pink, red, orange, blue, black, brown, yellow. The color often depends on the species, presence of symbiotic algae, and post-mortem alteration.
- Luster
- Dull to earthy, sometimes waxy when polished.
- Texture
- Porous, skeletal, often exhibiting intricate branching, massive, or plate-like structures. Can be rough or smooth depending on the degree of erosion and polishing.
- Crystal Form
- Microcrystalline to cryptocrystalline aggregates of aragonite or calcite, forming complex skeletal structures rather than distinct macroscopic crystals.
- Cleavage
- No distinct cleavage due to its aggregate nature. Individual aragonite or calcite crystals within the coral structure would exhibit cleavage, but the overall rock does not.
- Geological Environment
- Warm, shallow, clear marine waters, typically within the photic zone (where sunlight penetrates) for hermatypic (reef-building) corals. Found in tropical and subtropical oceans, forming extensive reef systems.
Key Facts
- Hardness: 3 - 4 on the Mohs scale (for aragonite/calcite components)
- Specific Gravity: 2.6 - 2.7 (variable due to porosity)
- Crystal System: Orthorhombic (for aragonite) or Trigonal (for calcite)
- Color: Highly variable; white, pink, red, orange, blue, black, brown, yellow
- Luster: Dull to earthy, waxy when polished
- Transparency: Opaque to translucent
- Fracture: Uneven to conchoidal (on individual skeletal elements)
- Cleavage: None (as a rock); individual aragonite/calcite crystals have distinct cleavage
- Composition: Primarily Calcium Carbonate (CaCO3), often with minor organic matter and other impurities.
Quick Check
- Color: Variable (white, pink, red, orange, blue, black, brown, yellow)
- Luster: Dull to earthy, sometimes waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, branching, columnar, encrusting, or plate-like skeletal structures, reflecting the growth form of the coral polyps.
- Cleavage Type: Not applicable for the rock as a whole; individual aragonite crystals exhibit distinct cleavage (pinacoidal {010} and prismatic {110}), while calcite exhibits perfect rhombohedral cleavage {1011}.
- Fracture Type: Uneven to splintery, sometimes conchoidal on dense, polished pieces.
- Tenacity: Brittle
- Luster Type: Dull to earthy, waxy when polished
Formation
Coral is formed from the skeletal remains of marine invertebrates called coral polyps. These polyps extract calcium carbonate (CaCO3) from seawater to build their hard exoskeletons. Over time, as polyps die, their skeletons accumulate and cement together, forming massive structures known as coral reefs. The primary mineral component is aragonite, a metastable polymorph of calcium carbonate, which can recrystallize into calcite over geological time.
Usage
Historically and presently used as a gemstone (especially precious corals like red coral), for decorative objects, in jewelry, and as a source of building material (coral aggregate, coral stone). In some regions, fossil coral is used as a dimension stone. Ecologically, living coral reefs are vital marine habitats and provide coastal protection.
Age Distribution
Cenozoic to Recent, with fossil corals dating back to the Ordovician period.
Where to Find
Great Barrier Reef, Australia
The world's largest coral reef system, extending over 2,300 kilometers, showcasing immense diversity of coral types.
Caribbean Sea
Home to numerous coral reefs, including those off the coasts of Belize, Florida (USA), and various Caribbean islands.
Indo-Pacific Region
Includes the Coral Triangle (Indonesia, Philippines, Malaysia, Papua New Guinea, Solomon Islands, Timor-Leste), a global center of marine biodiversity and coral reef abundance.
Red Sea
Known for its vibrant coral reefs and unique marine ecosystems.
Finding Tips
Ethical Collection
Collecting live coral is illegal and highly damaging to marine ecosystems. Only collect fossilized coral or coral fragments found on beaches, ensuring local regulations are followed. Many countries have strict laws against collecting any coral, live or dead.
Beachcombing
Look for coral fragments washed ashore on beaches in tropical and subtropical regions. These are typically weathered and may show less vibrant colors than live coral.
Fossil Localities
Fossil coral can be found in ancient marine sedimentary rock formations, often in limestone quarries or outcrops that were once ancient reef environments. These can be identified by their characteristic skeletal patterns preserved in the rock.
Commercial Sources
For jewelry or decorative pieces, purchase from reputable dealers who source coral ethically and legally, often from sustainable aquaculture or fossilized deposits.
Similar Rocks
Limestone
Limestone
Also known as: Calcium Carbonate Rock
Coquina
Coquina
Also known as: Shell Limestone
Chalk
Chalk
Also known as: Micritic Limestone
Scientific Classification
- Mineral Class
- Carbonate (as it is composed of calcium carbonate minerals)
- Group
- Biogenic Sedimentary Rock
- Crystal System
- Orthorhombic (aragonite) or Trigonal (calcite)
- Chemical Formula
- CaCO3
- Composition
- Calcium Carbonate (CaCO3), with minor organic components and trace elements.
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