How to identify Fossilized Coral

To correctly identify Fossilized Coral (Coral (calcium carbonate)), 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 with contrasting patterns from the original coral structure. Colors are influenced by trace minerals present during silicification.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to dull, sometimes vitreous (glassy) on polished surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Microcrystalline to cryptocrystalline, often with a smooth feel when polished. Unpolished surfaces can be rough or granular. The defining characteristic is the preserved internal structure of the coral, visible as intricate patterns.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Does not exhibit macroscopic crystal forms as it is cryptocrystalline silica. The 'form' is dictated by the original coral morphology.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is cryptocrystalline silica (chalcedony).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in ancient marine sedimentary deposits, particularly in areas that were once shallow, warm seas conducive to coral reef growth. These include limestones, shales, and sandstones that have undergone diagenesis and silicification. Often associated with other marine fossils.

Key Facts

Hardness

7 (Mohs scale, due to silicification)

Specific Gravity

2.58 - 2.64

Crystal System

Trigonal (for the constituent quartz/chalcedony)

Color

Highly variable, often with distinct coral patterns

Luster

Waxy to dull, sometimes vitreous

Transparency

Opaque to translucent

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Silicon dioxide (SiO2), with trace impurities

Physical characteristics

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

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