How to identify Fossilized Coral

To correctly identify Fossilized Coral (Rugosa or Tabulata coral (likely a type of Favosites or Hexagonaria)), 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, depending on the replacing minerals and impurities. Common colors include shades of gray, brown, white, black, pink, red, yellow, and blue. Often exhibits banding or concentric patterns within the individual corallites.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to waxy, especially on polished surfaces, due to the chalcedony replacement.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Microcrystalline to cryptocrystalline, smooth when polished. Unpolished surfaces may show a rougher, sometimes porous texture, revealing the original coral structure.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Does not exhibit macroscopic crystal forms; rather, it is a pseudomorph after coral, meaning it retains the external and internal structure of the original coral organism. The replacing silica is microcrystalline (chalcedony).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its microcrystalline nature. It exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in ancient marine sedimentary rock formations that were once shallow, warm, clear-water environments conducive to coral growth. These include limestones, shales, and sandstones, particularly those of Paleozoic age. The presence of silica-rich groundwater is crucial for the fossilization process.

Key Facts

Hardness

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

Specific Gravity

2.58-2.64 (typical for chalcedony/agate).

Crystal System

Trigonal (for the replacing chalcedony/quartz, though the overall form is pseudomorphic).

Color

Highly variable, often shades of gray, brown, white, black, pink, red, yellow, blue, with distinct patterns.

Luster

Vitreous to waxy.

Transparency

Opaque to translucent.

Fracture

Conchoidal.

Cleavage

None.

Composition

Primarily Silicon Dioxide (SiO2) in the form of chalcedony or agate, replacing original Calcium Carbonate (CaCO3).

Physical characteristics

• Crystal Habit: Pseudomorphic after coral skeletons; microcrystalline aggregates of quartz.
• Cleavage Type: None.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy.

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