How to identify Fossil Coral

To correctly identify Fossil Coral (Fossilized 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, ranging from white, gray, brown, black, yellow, orange, red, pink, to blue, often with multiple colors and patterns within a single specimen. Colors are influenced by mineral impurities during silicification.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to vitreous (glassy) when polished, dull to earthy in its raw, unpolished state.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth and often translucent when polished; rough and porous in its natural state. The surface may reveal the preserved cellular or skeletal structures of the original coral.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline, meaning the individual quartz crystals are too small to be seen without high magnification. The macroscopic form retains 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 deposits, particularly in areas that were once shallow, warm seas conducive to coral reef growth. These deposits are often associated with limestone, shale, and sandstone formations, and may be found in uplifted marine terraces or riverbeds that have eroded through such formations.

Key Facts

Hardness

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

Specific Gravity

2.58 - 2.64

Crystal System

Trigonal (for the constituent quartz/chalcedony)

Color

Highly variable, often multi-colored with distinct patterns

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 aggregates, preserving the original biogenic structure of the coral.
• Cleavage Type: Absent
• Fracture Type: Conchoidal, typical of cryptocrystalline quartz.
• Tenacity: Brittle
• Luster Type: Waxy to vitreous

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