How to identify Fossilized Coral

To correctly identify Fossilized Coral (Fossilized colonial coral (e.g., Favosites or Hexagonaria)), 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, including shades of gray, brown, white, black, pink, red, and orange, often with contrasting patterns from the original coral structure.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy, depending on the degree of silicification and polish.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth when polished; rough to slightly granular in its natural state. Distinctive patterns of corallites (individual coral polyps) or septa (internal divisions) are often visible.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (microscopic crystals) when silicified; original coral structure is preserved as a pseudomorph.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its cryptocrystalline nature.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in marine sedimentary rock formations, particularly limestones, shales, and sandstones, that were deposited in ancient shallow, warm, clear marine environments where coral reefs thrived. Often associated with ancient reef complexes or shallow shelf environments.

Key Facts

Hardness

6.5-7 on Mohs scale (when silicified); 3-4 (when calcified)

Specific Gravity

2.58-2.64 (when silicified); 2.71 (when calcified)

Crystal System

Trigonal (for quartz/chalcedony replacement); Orthorhombic/Hexagonal (original aragonite/calcite, but typically replaced)

Color

Highly variable, often patterned

Luster

Vitreous to waxy

Transparency

Opaque to translucent

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Primarily silicon dioxide (SiO2) when silicified; calcium carbonate (CaCO3) when calcified or original skeleton

Physical characteristics

• Crystal Habit: Pseudomorphic after coral structures; cryptocrystalline aggregates (chalcedony)
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to waxy

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