How to identify Fossilized Coral

To correctly identify Fossilized Coral (Coral (various species)), 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 white, gray, brown, black, pink, red, blue, and combinations thereof. Colors are often arranged in patterns reflecting the original coral structure.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy when polished; dull to earthy on unpolished surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular when polished. Unpolished surfaces may show fine details of the coral structure. Often exhibits a distinctive pattern of corallites, septa, or tabulae.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (microscopic crystals of quartz/chalcedony) replacing the original biogenic structure. No macroscopic crystal form of the replacing mineral is typically visible.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its cryptocrystalline nature and replacement origin. It exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in ancient marine sedimentary deposits, particularly limestones, shales, and sandstones that were once shallow, warm, clear-water environments conducive to coral growth. Often associated with fossiliferous beds and reef complexes.

Key Facts

Hardness

6.5-7 on the Mohs scale (due to silicification)

Specific Gravity

2.58-2.64

Crystal System

Trigonal (for the replacing chalcedony/quartz)

Color

Highly variable (white, gray, brown, black, pink, red, blue)

Luster

Vitreous to waxy

Transparency

Translucent to opaque

Fracture

Conchoidal

Cleavage

None

Composition

Primarily silicon dioxide (SiO2) replacing calcium carbonate (CaCO3)

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates, pseudomorphing original coral structures.
• Cleavage Type: None
• Fracture Type: Conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to waxy

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