How to identify Coral

To correctly identify Coral (Scleractinia (calcium carbonate skeleton)), 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

   Skeletal coral is typically white, off-white, or grey when dead and bleached. Living corals exhibit a wide range of colors (brown, green, blue, red, yellow, purple) due to symbiotic algae and pigments in the polyps. Fossil corals can be various shades of brown, grey, or black depending on diagenesis and mineral infilling.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy on unpolished surfaces; vitreous to waxy on polished surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Highly variable, reflecting the growth form of the coral colony. Can be porous, rough, smooth, branching, massive, or brain-like. Individual corallites (cups where polyps reside) are often visible, showing radial septa.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Microcrystalline to cryptocrystalline aggregates of aragonite or calcite. Individual crystals are typically too small to be seen without magnification. The overall form is biogenic, reflecting the colonial growth habit.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed macroscopically due to its cryptocrystalline nature and biogenic origin. Individual aragonite/calcite crystals within the skeleton would exhibit perfect rhombohedral cleavage, but this is not apparent in the bulk material.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Modern corals thrive in warm, shallow, clear, nutrient-poor marine waters, typically within the photic zone (0-70 meters depth, though some deep-sea corals exist). They are found in tropical and subtropical oceans. Fossil corals are found in ancient marine sedimentary sequences, often forming reef structures or dispersed within limestones, shales, and sandstones, indicating past marine environments.

Key Facts

Hardness

3-4 on Mohs scale (for aragonite/calcite)

Specific Gravity

2.7-2.9 (for aragonite/calcite)

Crystal System

Orthorhombic (aragonite) or Trigonal (calcite)

Color

White, off-white, grey (skeletal); diverse colors (living)

Luster

Dull to earthy, vitreous to waxy

Transparency

Opaque

Fracture

Uneven to conchoidal

Cleavage

None observed macroscopically (perfect rhombohedral for constituent calcite/aragonite crystals)

Composition

Calcium Carbonate (CaCO3), primarily aragonite

Physical characteristics

• Crystal Habit: Biogenic, colonial, forming massive, branching, encrusting, or foliaceous structures. Composed of microcrystalline to cryptocrystalline aragonite/calcite.
• Cleavage Type: Not applicable to the bulk biogenic material; constituent aragonite/calcite crystals have perfect rhombohedral cleavage.
• Fracture Type: Uneven to conchoidal, depending on the density and structure of the coral skeleton.
• Tenacity: Brittle
• Luster Type: Dull to earthy on natural surfaces; vitreous to waxy when polished.

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