Coral

Description

Coral refers to the skeletal remains of marine polyps, primarily scleractinian (hard) corals, which are colonial organisms. These skeletons are composed predominantly of calcium carbonate, typically in the aragonite polymorph, though some ancient corals and certain modern species may incorporate calcite. The macroscopic structure reflects the growth patterns of the coral colony, often exhibiting intricate, branching, massive, or plate-like forms. Living corals are vibrant, but the skeletal material, when dead and cleaned, is typically white or off-white, though impurities or specific species can impart colors such as red, pink, blue, or black.

How to Identify

Color

Typically white, off-white, or cream when dead and cleaned. Living corals exhibit a wide range of colors due to symbiotic algae (zooxanthellae) and pigments. Precious corals (e.g., Corallium rubrum) are naturally red, pink, or orange. Black coral (Antipatharia) is dark brown to black.

Luster

Dull to vitreous (glassy) on fractured surfaces, often earthy or porous on natural surfaces.

Texture

Highly variable, reflecting the colonial growth form. Can be porous, spongy, massive, branching, columnar, or plate-like. Often exhibits distinct septa (radial partitions) and corallites (individual polyp cups) visible with magnification.

Crystal Form

Microcrystalline to cryptocrystalline aggregates of aragonite or calcite. Macroscopically, the form is biogenic, reflecting the coral colony's morphology.

Cleavage

No macroscopic cleavage due to its aggregate nature. Individual aragonite or calcite crystals would exhibit perfect rhombohedral cleavage (calcite) or distinct cleavage (aragonite), but these are microscopic within the coral structure.

Geological Environment

Warm, shallow, clear marine waters, typically within the photic zone (where sunlight penetrates) for hermatypic (reef-building) corals. Ahermatypic (non-reef-building) corals can occur in deeper, colder waters.

Key Facts

• Hardness: 3-4 on Mohs scale (for aragonite/calcite, but aggregate structure can make it seem softer)
• Specific Gravity: 2.6-2.9 (variable due to porosity and mineralogy)
• Crystal System: Orthorhombic (aragonite) or Trigonal (calcite) for individual crystals; macroscopically amorphous/biogenic aggregate.
• Color: White, cream, red, pink, black, blue (depending on species and preservation)
• Luster: Dull, earthy, sometimes vitreous on fresh breaks
• Transparency: Opaque to translucent
• Fracture: Uneven to conchoidal (on dense varieties)
• Cleavage: None observed macroscopically; microscopic aragonite/calcite crystals have distinct cleavage.
• Composition: Calcium carbonate (CaCO3), primarily aragonite, with minor calcite and organic matter.

Quick Check

• Color: White, off-white, cream (dead); various colors (living); red, pink, black (precious/specific types)
• Luster: Dull to vitreous (fractured), earthy/porous (natural surface)
• Streak: White

Physical Characteristics

• Crystal Habit: Biogenic, colonial structures: branching, massive, encrusting, foliaceous, columnar, solitary (e.g., horn corals).
• Cleavage Type: Not applicable to the aggregate rock; microscopic aragonite has distinct cleavage, calcite has perfect rhombohedral cleavage.
• Fracture Type: Uneven, splintery, or conchoidal (in dense varieties).
• Tenacity: Brittle
• Luster Type: Dull to earthy on natural surfaces; vitreous on fresh, dense fractures.

Formation

Coral is formed by colonies of marine invertebrates called polyps (phylum Cnidaria, class Anthozoa). These polyps extract calcium and carbonate ions from seawater to secrete an external skeleton of calcium carbonate (CaCO3). Over time, the accumulation of these skeletons, often cemented by other biogenic and inorganic processes, forms coral reefs and eventually coral rock. The primary mineral phases are aragonite and, to a lesser extent, calcite.

Usage

Historically and presently used as a gemstone and ornamental material (e.g., 'red coral' or 'precious coral' from the genus Corallium). Fossil corals are collected by enthusiasts. In some regions, coral rock is quarried as a building material or for agricultural lime. Modern scientific research utilizes coral skeletons for paleoclimate reconstruction due to their growth rings and isotopic signatures.

Age Distribution

Phanerozoic Eon (Cambrian to Recent), with significant reef-building corals appearing in the Ordovician and flourishing from the Mesozoic to Cenozoic.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Carbonate (as the primary mineral component)

Group

Biogenic Sedimentary Rock

Crystal System

Orthorhombic (aragonite) / Trigonal (calcite)

Chemical Formula

CaCO3

Composition

Calcium carbonate, primarily aragonite, with minor calcite and organic components. Trace elements can be incorporated from seawater.

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