How to identify Fossil Coral

To correctly identify Fossil Coral (Fossilized Rugose or Tabulate Coral (Chalcedony/Agate)), 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: white, gray, brown, black, yellow, pink, red, blue, often with contrasting patterns from the original coral structure. Colors are due to trace mineral inclusions.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to vitreous (glassy) when polished, dull to earthy on unpolished surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular, often exhibiting the preserved intricate patterns of the original coral colony (e.g., hexagonal cells, radial lines, septa, tabulae).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline to microcrystalline quartz (chalcedony/agate), forming botryoidal, mammillary, or massive aggregates that replace the original coral structure. No macroscopic crystal forms are typically visible.

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 sedimentary rock formations that were once ancient marine environments where corals thrived. These environments were subsequently subjected to burial and silicification processes. Often associated with limestones, shales, and sandstones, particularly in areas with past volcanic activity providing silica-rich waters.

Key Facts

Hardness

6.5-7 on the Mohs scale (due to chalcedony/agate replacement).

Specific Gravity

2.58-2.64 (typical for chalcedony/agate).

Crystal System

Trigonal (for the constituent quartz/chalcedony).

Color

Highly variable, often patterned, reflecting original coral structure.

Luster

Waxy to vitreous.

Transparency

Translucent to opaque.

Fracture

Conchoidal.

Cleavage

None.

Composition

Silicon dioxide (SiO2), with trace impurities causing color variations. Original coral was Calcium Carbonate (CaCO3).

Physical characteristics

• Crystal Habit: Massive, cryptocrystalline to microcrystalline aggregates, pseudomorphing original coral structures.
• Cleavage Type: None.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Waxy to vitreous.

Explore Fossil Coral