How to identify Fossilized Coral

To correctly identify Fossilized Coral (Fossilized Coral (Anthozoa)), 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, orange, yellow, and blue. Often exhibits multiple colors in intricate patterns.

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 when polished; rough to slightly granular on unpolished surfaces. Internal structures may create a slightly uneven texture.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (chalcedony) or microcrystalline (quartz/calcite) replacement of original coral structures. No macroscopic crystal form of the replacing mineral is typically observed, but the original coral morphology is preserved.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its cryptocrystalline nature (chalcedony) or the lack of a distinct cleavage plane in the original coral structure. If replaced by calcite, it may exhibit rhombohedral cleavage, but this is rare for well-preserved fossils.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine sedimentary environments, particularly ancient shallow seas, reefs, and lagoons where corals thrived. Found in sedimentary rock formations, often associated with limestones, shales, and sandstones.

Key Facts

Hardness

6.5-7 (Mohs scale) if silicified (chalcedony/quartz); 3 if primarily calcite.

Specific Gravity

2.58-2.64 (if silicified); 2.71 (if calcitic).

Crystal System

Trigonal (for replacing quartz/chalcedony); Rhombohedral (for replacing calcite). The original coral skeleton was amorphous or aragonitic (orthorhombic).

Color

Highly variable: white, gray, brown, black, pink, red, orange, yellow, blue, often with intricate patterns.

Luster

Vitreous to waxy.

Transparency

Opaque to translucent.

Fracture

Conchoidal to uneven.

Cleavage

None (for silicified coral); perfect rhombohedral (for calcitic replacement, but rare in well-preserved fossils).

Composition

Primarily Silicon Dioxide (SiO2) if agatized/silicified; Calcium Carbonate (CaCO3) if calcitic replacement.

Physical characteristics

• Crystal Habit: Pseudomorphic replacement of original coral structures; cryptocrystalline or microcrystalline aggregates.
• Cleavage Type: Absent (silicified); perfect rhombohedral (calcitic, rare).
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy.

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