How to identify Fossilized Coral

To correctly identify Fossilized Coral (Fossilized Scleractinian 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, pink, red, orange, yellow, to blue, often with multiple colors in a single specimen. The color is influenced by impurities (e.g., iron oxides for reds/browns, manganese for blacks).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to waxy, sometimes dull.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular when polished; unpolished surfaces can be rough. The most distinctive feature is the preserved internal structure of the coral, appearing as intricate patterns of circles, polygons, or radiating lines, representing the corallites, septa, and tabulae.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (microscopic crystals), typically chalcedony, which is a fibrous variety of quartz. No macroscopic crystal forms are visible.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its cryptocrystalline nature. It exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in marine sedimentary rock formations, particularly limestones, shales, and sandstones, that were deposited in ancient shallow, warm seas where coral reefs flourished. Often associated with other marine fossils.

Key Facts

Hardness

6.5-7 (Mohs scale, due to silicification)

Specific Gravity

2.58-2.64

Crystal System

Trigonal (for quartz/chalcedony component)

Color

Highly variable, often banded or mottled with preserved coral patterns

Luster

Vitreous to waxy

Transparency

Translucent to opaque

Fracture

Conchoidal

Cleavage

None

Composition

Primarily silicon dioxide (SiO2), with trace impurities causing color variations. Original coral was calcium carbonate (CaCO3).

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates, forming botryoidal, mammillary, or stalactitic masses, often filling voids or replacing organic structures.
• Cleavage Type: None
• Fracture Type: Conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to waxy

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