How to identify Fossil Coral

To correctly identify Fossil Coral (Rugosa or Tabulata coral fossil in limestone or chert), 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 from the preserved coral structures. The color is influenced by trace minerals during silicification or the composition of the surrounding matrix.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy when polished (chalcedony replacement), dull to earthy (calcite or unpolished).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Microcrystalline to cryptocrystalline (chalcedony replacement), granular (calcite replacement or limestone matrix). The surface often reveals intricate patterns of the original coral polyps, such as radial septa, concentric growth rings, or honeycomb-like structures.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable in the macroscopic sense, as it's a fossilized organism. Microscopically, chalcedony is cryptocrystalline, and calcite is microcrystalline to crystalline.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed macroscopically due to its cryptocrystalline or granular nature. Calcite components may exhibit perfect rhombohedral cleavage, but this is rarely visible in the fossilized coral structure itself.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine sedimentary environments, particularly ancient shallow seas, reefs, and lagoons. Found within limestone, chert beds, or silicified nodules where conditions were favorable for coral growth and subsequent fossilization.

Key Facts

Hardness

6.5-7 on Mohs scale (for silicified coral/chalcedony); 3 (for calcite-replaced coral or limestone matrix).

Specific Gravity

2.58-2.64 (for silicified coral); 2.71 (for calcite).

Crystal System

Trigonal (for chalcedony/quartz); Trigonal (for calcite). However, the fossil itself is an organic structure.

Color

Wide range, often patterned.

Luster

Vitreous to waxy (silicified), dull to earthy (calcite/unpolished).

Transparency

Opaque to translucent.

Fracture

Conchoidal (silicified); Uneven to subconchoidal (calcite/limestone).

Cleavage

None (silicified); Perfect rhombohedral (calcite, if present as distinct crystals).

Composition

Primarily SiO2 (chalcedony) or CaCO3 (calcite), preserving the original coral morphology.

Physical characteristics

• Crystal Habit: Cryptocrystalline (chalcedony) or microcrystalline to granular (calcite), preserving the original biogenic structure of the coral colony.
• Cleavage Type: Not applicable for the overall fossil structure; chalcedony has no cleavage, calcite has perfect rhombohedral cleavage.
• Fracture Type: Conchoidal (silicified) to uneven (calcite/limestone).
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy (silicified), dull to earthy (calcite/unpolished).

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