How to identify Coral

To correctly identify Coral (Coral (biogenic calcium carbonate)), 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

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

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. 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.

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.

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