How to identify Fossilized Coral

To correctly identify Fossilized Coral (Rugosa or Tabulata (extinct coral orders)), 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 mineral impurities present during silicification. Common colors include shades of gray, brown, white, 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 the silicification is very fine-grained and pure.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular when polished. The surface often reveals the intricate patterns of the original coral structure, such as septa, tabulae, and corallites. Can feel cool and dense to the touch.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Microcrystalline to cryptocrystalline quartz (chalcedony/agate) replacing the original coral structure. No macroscopic crystal forms are typically visible, but microscopic examination reveals fibrous or granular quartz.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is composed of microcrystalline quartz.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in marine sedimentary rock sequences, particularly limestones, shales, and sandstones that were deposited in ancient shallow marine environments where coral reefs thrived. Often associated with fossiliferous beds.

Key Facts

Hardness

6.5-7 on Mohs scale (due to silicification)

Specific Gravity

2.58-2.64 (typical for chalcedony/agate)

Crystal System

Trigonal (for quartz/chalcedony)

Color

Highly variable, often patterned

Luster

Waxy to dull, sometimes vitreous

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 to microcrystalline aggregates, pseudomorphous after coral skeletons.
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Waxy, dull, or vitreous

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