How to identify Brain Coral

To correctly identify Brain Coral (Scleractinia (Faviidae family)), 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 light gray when dead and bleached. Living corals can exhibit a wide range of colors (brown, green, yellow, blue) due to symbiotic algae (zooxanthellae) and pigments.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy on unpolished surfaces; vitreous to pearly on freshly broken or polished surfaces of the calcium carbonate skeleton.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rough, porous, and highly convoluted surface with distinct ridges (gyri) and valleys (sulci) that mimic the folds of a brain. The internal structure, if broken, reveals a dense, crystalline calcium carbonate matrix.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Macrocrystalline to cryptocrystalline aggregates of aragonite (orthorhombic) or calcite (trigonal) depending on diagenesis. The overall form is colonial, hemispherical, or spherical.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No macroscopic cleavage planes due to its biogenic, polycrystalline nature. Individual aragonite crystals within the skeleton would exhibit prismatic cleavage, but this is not observable in the coral structure itself.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Warm, shallow, clear marine waters of tropical and subtropical oceans, typically within coral reefs. Fossil brain corals are found in ancient marine sedimentary deposits, often associated with other fossil reef organisms.

Key Facts

Hardness

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

Specific Gravity

2.7-2.9 (for aragonite/calcite)

Crystal System

Orthorhombic (aragonite) or Trigonal (calcite) for the constituent minerals; overall colonial structure is amorphous/biogenic.

Color

White, off-white, light gray (dead); various colors (living)

Luster

Dull to earthy

Transparency

Opaque

Fracture

Uneven to conchoidal

Cleavage

None observable macroscopically

Composition

Calcium Carbonate (CaCO3), primarily aragonite, often recrystallized to calcite in fossils.

Physical characteristics

• Crystal Habit: Colonial, massive, hemispherical to spherical with convoluted ridges and valleys.
• Cleavage Type: Not applicable to the colonial structure; individual aragonite crystals have distinct cleavage.
• Fracture Type: Uneven to conchoidal, depending on the density and crystalline structure.
• Tenacity: Brittle
• Luster Type: Dull to earthy on the surface; vitreous to pearly on fresh breaks.

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