How to identify Fossil Coral

To correctly identify Fossil Coral (Fossilized Anthozoa (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, including white, gray, brown, black, pink, red, yellow, and blue. Often exhibits multiple colors in a single specimen due to mineral impurities during silicification.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy, depending on the degree of silicification and polish.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular when polished. Unpolished surfaces may show rougher textures reflecting the original coral structure. Distinctive patterns of preserved coral polyps are the key identifying feature.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (chalcedony/agate) or microcrystalline quartz, replacing the original coral structure. Does not exhibit macroscopic crystal forms of quartz.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its cryptocrystalline nature. It fractures conchoidally.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in marine sedimentary rock formations, particularly limestones, shales, and sandstones that were deposited in ancient shallow, warm seas where coral reefs thrived. Often associated with areas of past volcanic activity that provided silica-rich solutions for replacement.

Key Facts

Hardness

6.5-7 on Mohs scale (if silicified); 3-4 (if primarily calcium carbonate)

Specific Gravity

2.58-2.64 (if silicified); 2.71 (if primarily calcite)

Crystal System

Trigonal (for chalcedony/quartz replacement); Orthorhombic (for original aragonite); Hexagonal (for original calcite)

Color

Highly variable, often multi-colored with patterns

Luster

Vitreous to waxy

Transparency

Translucent to opaque

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Primarily SiO2 (silica, as chalcedony/agate) if silicified; CaCO3 (calcium carbonate, as calcite or aragonite) if un-silicified or partially replaced.

Physical characteristics

• Crystal Habit: Cryptocrystalline or microcrystalline aggregates, pseudomorphing the original coral structure.
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to waxy

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