How to identify Fossilized Coral

To correctly identify Fossilized Coral (Rugosa or Tabulata coral fossil), 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 trace minerals present during silicification. Common colors include white, gray, brown, black, pink, red, yellow, and blue. Often exhibits banding or concentric patterns.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy, especially when polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Microcrystalline to cryptocrystalline, often with a smooth feel when polished. Unpolished surfaces can be rough or granular. The defining characteristic is the preserved internal structure of the coral, visible as intricate patterns.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (chalcedony) or microcrystalline (quartz). Individual crystals are typically too small to be seen without magnification.

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

   Marine sedimentary environments, particularly ancient shallow seas, reefs, and lagoons where corals thrived. Found within limestone, shale, or sandstone formations that have undergone silicification.

Key Facts

Hardness

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

Specific Gravity

2.58-2.64 (for chalcedony/quartz)

Crystal System

Trigonal (for quartz/chalcedony)

Color

Highly variable, often with preserved coral patterns

Luster

Vitreous to waxy

Transparency

Translucent to opaque

Fracture

Conchoidal

Cleavage

None

Composition

Primarily silicon dioxide (SiO2) after silicification, with trace impurities causing color variations. Original coral was calcium carbonate (CaCO3).

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates, forming botryoidal, mammillary, or stalactitic masses, or filling voids and replacing organic structures.
• Cleavage Type: None
• Fracture Type: Conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to waxy

Explore Fossilized Coral