How to identify Fossilized Coral

To correctly identify Fossilized Coral (Coral (Anthozoa)), 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, ranging from white, gray, brown, black, pink, red, yellow, to blue. Often exhibits multiple colors in intricate patterns.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy, sometimes dull if unpolished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically smooth when polished, but can be rough or granular in its natural state. The surface often reveals the intricate patterns of the original coral structure.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. The overall form is dictated by the original coral morphology (e.g., massive, branching, columnar, tabulate, rugose).

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 sedimentary rock formations that were once ancient marine environments, particularly shallow, warm seas where coral reefs thrived. These include limestones, shales, and sandstones, often in areas that have undergone silicification.

Key Facts

Hardness

6.5 - 7 on the Mohs scale (for silicified coral)

Specific Gravity

2.58 - 2.64 (for silicified coral)

Crystal System

Trigonal (for chalcedony replacement)

Color

Wide range, often patterned

Luster

Vitreous to waxy

Transparency

Opaque to translucent

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Primarily silicon dioxide (SiO2) when silicified; originally calcium carbonate (CaCO3)

Physical characteristics

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

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