How to identify Fossil Coral

To correctly identify Fossil Coral (Silicified 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: white, gray, brown, black, pink, red, yellow, orange, often with multiple colors or banding. The color is due to impurities like iron oxides, manganese, or organic matter.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to vitreous (glassy) when polished, dull to earthy on unpolished or weathered surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular. The most distinguishing feature is the visible fossilized coral patterns (e.g., hexagonal, radial, or dendritic structures) on polished or cut surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Microcrystalline (chalcedony), so individual crystals are not visible to the naked eye. The macroscopic form is dictated by the original coral morphology.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in marine sedimentary rock formations where ancient coral reefs once thrived. These environments are typically shallow, warm, clear waters. Subsequent burial and diagenesis involving silica-rich fluids are necessary for silicification.

Key Facts

Hardness

6.5-7 on the Mohs scale (due to silicification)

Specific Gravity

2.58-2.64 (typical for chalcedony)

Crystal System

Trigonal (for quartz/chalcedony)

Color

Highly variable, often with visible coral patterns

Luster

Waxy to vitreous

Transparency

Opaque to translucent

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Silicon dioxide (SiO2), microcrystalline quartz (chalcedony), with trace impurities

Physical characteristics

• Crystal Habit: Microcrystalline aggregates, forming pseudomorphs after coral skeletons.
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven, splintery in some cases.
• Tenacity: Brittle
• Luster Type: Waxy to vitreous

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