How to identify Fossilized Coral

To correctly identify Fossilized Coral (Rugose Coral (Horn Coral) fossil in chert or limestone), 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

   Matrix color varies widely: Limestone can be white, gray, tan, brown, or black. Chert can be white, gray, black, brown, red, or green. The fossilized coral itself often appears as a lighter or darker contrast to the matrix, or can be the same color if completely replaced by the matrix material. Silicified corals often show a translucent to opaque appearance.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Matrix: Limestone is typically dull to earthy. Chert is typically waxy to dull, sometimes vitreous (glassy) on fresh fracture. The fossilized coral structure within chert can exhibit a waxy to vitreous luster when polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Matrix: Limestone can be fine-grained to coarse-grained, often with a gritty or smooth feel. Chert is typically very fine-grained (cryptocrystalline) and smooth to the touch, with a conchoidal fracture. The fossil itself will show the characteristic morphology of the coral, such as radial septa and concentric growth lines, which can be felt as subtle ridges or seen as patterns.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   The fossil itself retains the original biological form of the Rugose coral, which is typically solitary and horn-shaped (conical to cylindrical), or less commonly colonial. The internal structure includes radial septa and horizontal tabulae. The matrix (chert or limestone) is cryptocrystalline (chert) or microcrystalline to macrocrystalline (limestone) and does not exhibit distinct crystal forms unless it's a large calcite crystal.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Limestone (calcite) exhibits perfect rhombohedral cleavage (3 directions not at 90 degrees). Chert has no cleavage, exhibiting conchoidal fracture. The fossilized coral structure itself does not exhibit cleavage, but rather fracture along its preserved biological structures.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine sedimentary environments, particularly shallow, warm seas where coral reefs or isolated coral growth flourished. Found in ancient marine limestones, shales, and chert beds that formed from the diagenesis of silica-rich sediments or replacement of carbonate sediments.

Key Facts

Hardness

3 (for calcite in limestone) to 7 (for quartz in chert) on the Mohs scale. The fossil itself will have the hardness of the material it is composed of or replaced by.

Specific Gravity

2.6-2.8 (for calcite/limestone) to 2.6-2.7 (for quartz/chert).

Crystal System

Trigonal (for calcite) or Hexagonal (for quartz). The fossil retains the biological form, not a mineral crystal system.

Color

Highly variable, depending on the matrix and diagenetic alteration. Common colors include grays, browns, whites, and blacks.

Luster

Dull to waxy (chert), earthy (limestone), waxy to vitreous (polished silicified coral).

Transparency

Opaque to translucent (especially silicified specimens).

Fracture

Conchoidal (chert), uneven to splintery (limestone). The fossil itself will fracture according to its preserved structure and composition.

Cleavage

Perfect rhombohedral (calcite in limestone), none (chert).

Composition

Primarily Calcium Carbonate (CaCO3) in limestone, or Silicon Dioxide (SiO2) when silicified in chert. Trace elements can vary depending on the original organism and diagenetic fluids.

Physical characteristics

• Crystal Habit: Not applicable to the fossil itself, which is a biogenic structure. The matrix minerals (calcite or quartz) have their own habits (e.g., rhombohedral for calcite, microcrystalline for chert).
• Cleavage Type: Perfect rhombohedral (calcite), none (chert).
• Fracture Type: Conchoidal (chert), uneven to splintery (limestone).
• Tenacity: Brittle.
• Luster Type: Dull, earthy, waxy, or vitreous.

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