How to identify Fossilized Scleractinian Coral

To correctly identify Fossilized Scleractinian Coral (Fossilized Scleractinian 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, yellow, to blue, often with concentric banding or mottled patterns 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 when polished; rough to granular on unpolished surfaces. The defining characteristic is the preserved internal structure of the coral, such as septa, corallites, and colonial patterns, which can be seen on cut or polished surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is composed of cryptocrystalline silica. It exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in ancient marine sedimentary deposits, particularly in areas that were once shallow, warm seas conducive to coral reef growth. These include limestones, shales, and sandstones that have undergone diagenesis and silicification.

Key Facts

Hardness

6.5-7 on the Mohs scale (due to silicification). Original coral (aragonite/calcite) is 3-4.

Specific Gravity

2.58-2.64 (for agatized coral).

Crystal System

Trigonal (for the replacing quartz/chalcedony). The original coral skeleton is orthorhombic (aragonite) or trigonal (calcite).

Color

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

Luster

Vitreous to waxy.

Transparency

Opaque to translucent.

Fracture

Conchoidal.

Cleavage

None.

Composition

Primarily silicon dioxide (SiO2) for agatized coral, with trace impurities. Original coral was calcium carbonate (CaCO3).

Physical characteristics

• Crystal Habit: Cryptocrystalline or microcrystalline aggregates, preserving the original biogenic structure of the coral.
• Cleavage Type: Absent.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy.

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