How to identify Fossil Coral

To correctly identify Fossil Coral (Fossilized Rugose or Tabulate 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, ranging from white, gray, brown, black, yellow, orange, red, to pink, often with multiple colors in a single specimen due to impurities and varying mineralization. The patterns are key.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to vitreous (glassy) when polished, dull to earthy in its natural, unpolished state.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth and cool to the touch when polished. Unpolished specimens may feel slightly rough or granular. The internal structure often presents a distinctive pattern.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (microscopic crystals), forming botryoidal, mammillary, or banded structures within the coral's original framework. No macroscopic crystal forms are typically visible.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is composed of cryptocrystalline quartz (chalcedony). It exhibits conchoidal fracture.

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 that have undergone silicification.

Key Facts

Hardness

6.5-7 on the Mohs scale (due to silica replacement).

Specific Gravity

2.58-2.64 (typical for chalcedony/agate).

Crystal System

Trigonal (for the replacing quartz/chalcedony).

Color

Highly variable: white, gray, brown, black, yellow, orange, red, pink, often multi-colored.

Luster

Waxy to vitreous.

Transparency

Opaque to translucent.

Fracture

Conchoidal to uneven.

Cleavage

None.

Composition

Primarily silicon dioxide (SiO2) in the form of chalcedony or agate, replacing original calcium carbonate (CaCO3).

Physical characteristics

• Crystal Habit: Pseudomorph after coral, cryptocrystalline aggregates.
• Cleavage Type: None.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Waxy to vitreous.

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