How to identify Coral

To correctly identify Coral (Calcium carbonate (biogenic)), 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. Often uniform within a single specimen, but can be mottled or banded.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to waxy or vitreous when polished; typically dull to earthy on unpolished surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porous, granular, fibrous, or compact, depending on the coral species and preservation. Often exhibits characteristic growth patterns (e.g., septa, corallites, growth rings).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Microcrystalline to cryptocrystalline aggregates of aragonite or calcite. Individual crystals are rarely visible without magnification.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed in macroscopic coral structures due to its biogenic, aggregate nature. The constituent aragonite or calcite crystals would have perfect rhombohedral cleavage, but this is not apparent in the overall 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 be found in deeper, colder waters. Fossil corals are found in ancient marine sedimentary environments, often forming limestones.

Key Facts

Hardness

3 to 4 on the Mohs scale (due to aragonite/calcite composition)

Specific Gravity

2.6 to 2.7 (variable depending on porosity and composition)

Crystal System

Orthorhombic (for aragonite) or Trigonal (for calcite), but coral itself is an aggregate.

Color

White, pink, red, orange, blue, black, brown, yellow

Luster

Dull, earthy, waxy, or vitreous (when polished)

Transparency

Opaque to translucent

Fracture

Uneven to conchoidal (in dense varieties)

Cleavage

None observed in bulk coral; constituent minerals have perfect rhombohedral (calcite) or distinct (aragonite) cleavage.

Composition

Calcium carbonate (CaCO3), primarily aragonite, with minor organic matter and trace elements.

Physical characteristics

• Crystal Habit: Massive, branching, columnar, encrusting, foliaceous, or solitary (depending on species). Composed of microcrystalline aggregates.
• Cleavage Type: Not applicable to the bulk material; constituent aragonite has distinct cleavage, calcite has perfect rhombohedral cleavage.
• Fracture Type: Uneven, splintery, or conchoidal (in dense varieties).
• Tenacity: Brittle
• Luster Type: Dull to waxy or vitreous (when polished)

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