How to identify Coral

To correctly identify Coral (Coral (biogenic limestone)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes waxy when polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porous, skeletal, often exhibiting intricate branching, massive, or plate-like structures. Can be rough or smooth depending on the degree of erosion and polishing.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Microcrystalline to cryptocrystalline aggregates of aragonite or calcite, forming complex skeletal structures rather than distinct macroscopic crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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

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