How to identify Fossilized Coral

To correctly identify Fossilized Coral (Rugosa or Tabulata (extinct orders of coral)), 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, depending on the trace minerals present during silicification. Common colors include white, gray, brown, black, pink, red, yellow, and blue. Often exhibits multiple colors in banded or mottled patterns.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to dull, sometimes vitreous (glassy) if highly polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular. The surface often reveals the intricate patterns of the original coral structure.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (microscopic crystals of quartz/chalcedony), preserving the macroscopic form of the original coral colony or solitary polyp.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is composed of cryptocrystalline quartz/chalcedony.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in marine sedimentary rock formations that were once ancient coral reefs. These environments are typically shallow, warm, clear waters conducive to coral growth, followed by burial and silicification.

Key Facts

Hardness

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

Specific Gravity

2.58-2.64 (similar to quartz/chalcedony)

Crystal System

Trigonal (for the replacing quartz/chalcedony)

Color

Highly variable, often banded or mottled

Luster

Waxy to dull

Transparency

Opaque to translucent

Fracture

Conchoidal to uneven

Cleavage

None

Composition

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

Physical characteristics

• Crystal Habit: Cryptocrystalline, preserving original coral morphology
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Waxy to dull

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