How to identify Coral

To correctly identify Coral (Anthozoa (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, but can be naturally colored red, pink, orange, blue, black, or brown due to organic pigments or mineral inclusions. Fossil coral, especially agatized varieties, can exhibit a wide range of colors from white, gray, brown, to black, often with distinct fossil patterns.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to waxy or vitreous (when polished or agatized).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porous, often with visible colonial structures, septa, and corallites. Fossil coral may retain these textures, sometimes replaced by fine-grained silica.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Microcrystalline to cryptocrystalline aggregates of aragonite or calcite. No macroscopic crystal forms are typically observed in the skeletal material itself, but the overall structure reflects the colonial growth habit.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed in the aggregate skeletal material. Individual aragonite or calcite crystals would exhibit cleavage, but the biogenic structure prevents macroscopic observation.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Modern coral is found in shallow, warm, clear, nutrient-poor marine waters, typically in tropical and subtropical regions, forming coral reefs. Fossil coral is found in marine sedimentary rock sequences, often within limestones, shales, or sandstones, indicating ancient marine environments.

Key Facts

Hardness

3-4 on Mohs scale (for aragonite/calcite skeleton); 6.5-7 for agatized fossil coral (chalcedony)

Specific Gravity

2.6-2.7 (for aragonite/calcite skeleton); 2.58-2.64 for agatized fossil coral

Crystal System

Orthorhombic (aragonite) or Trigonal (calcite); Cryptocrystalline (chalcedony in agatized coral)

Color

White, red, pink, orange, blue, black, brown (modern); varied, often patterned (fossil)

Luster

Dull to waxy or vitreous

Transparency

Opaque to translucent

Fracture

Uneven to splintery (skeletal); Conchoidal (agatized fossil coral)

Cleavage

None observed macroscopically

Composition

Calcium Carbonate (CaCO3), primarily aragonite, sometimes calcite. Agatized fossil coral is primarily Silicon Dioxide (SiO2).

Physical characteristics

• Crystal Habit: Colonial, massive, branching, plate-like, or solitary forms, reflecting the growth of polyps. Microcrystalline aggregates.
• Cleavage Type: Not applicable to the aggregate biogenic structure.
• Fracture Type: Uneven to splintery for the skeletal material; conchoidal for agatized varieties.
• Tenacity: Brittle
• Luster Type: Dull to waxy (unpolished); vitreous (polished or agatized)

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