How to identify Fossilized Coral

To correctly identify Fossilized Coral (Fossilized Scleractinian or Tabulate Coral (likely Hexagonaria percarinata)), 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. The color is influenced by trace minerals present during silicification.

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 and dense when polished. Unpolished surfaces may feel slightly granular or rough. The defining characteristic is the visible internal structure of the coral, such as hexagonal patterns (Hexagonaria), radiating septa, or concentric growth rings.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (microscopic crystals of quartz), forming a massive, dense aggregate that preserves the original coral morphology.

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 ancient marine sedimentary rock formations that once hosted coral reefs. These environments are typically shallow, warm, clear marine waters. After burial, groundwater rich in dissolved silica infiltrates the coral skeletons, leading to their replacement.

Key Facts

Hardness

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

Specific Gravity

2.58-2.64 (typical for chalcedony/agate).

Crystal System

Trigonal (for the constituent quartz/chalcedony).

Color

Highly variable, often with distinct patterns of the original coral structure.

Luster

Waxy to vitreous.

Transparency

Opaque to translucent.

Fracture

Conchoidal.

Cleavage

None.

Composition

Primarily silicon dioxide (SiO2), replacing original calcium carbonate (CaCO3).

Physical characteristics

• Crystal Habit: Cryptocrystalline, massive, preserving the original biogenic structure of the coral.
• Cleavage Type: None.
• Fracture Type: Conchoidal, typical of quartz.
• Tenacity: Brittle.
• Luster Type: Waxy to vitreous.

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