How to identify Fossilized Coral (Tabulate Coral)

To correctly identify Fossilized Coral (Tabulate Coral) (Tabulate Coral (e.g., Favosites)), 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 due to mineral impurities (e.g., iron oxides for reds/browns, manganese for blacks).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy, depending on the degree of silicification and polish.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth when polished; rough to granular on unpolished surfaces. The characteristic texture is the visible pattern of individual corallites, often polygonal (hexagonal in Favosites) or circular, with internal structures like tabulae or septa visible.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (chalcedony/chert) or microcrystalline calcite, preserving the original biogenic structure of the coral colony. Not a true crystal form in the mineralogical sense, but rather a pseudomorph after the coral skeleton.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is typically composed of cryptocrystalline quartz or microcrystalline calcite. Fracture is conchoidal to uneven.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in marine sedimentary rock formations, particularly limestones, shales, and cherts, that were deposited in ancient shallow, warm, clear marine environments conducive to coral growth. Often associated with ancient reef complexes.

Key Facts

Hardness

6.5-7 on Mohs scale (if silicified as chalcedony/chert); 3-4 (if primarily calcitic). Most collectible fossil corals are silicified.

Specific Gravity

2.58-2.64 (for silicified varieties); 2.71 (for calcitic varieties).

Crystal System

Trigonal (for chalcedony/quartz) or Hexagonal (for calcite), but the material is cryptocrystalline or microcrystalline, preserving the original biogenic structure.

Color

Wide range: white, gray, brown, black, pink, red, yellow, blue, often mottled or banded.

Luster

Vitreous to waxy.

Transparency

Opaque to translucent.

Fracture

Conchoidal to uneven.

Cleavage

None (for silicified); perfect rhombohedral (for calcitic, but rarely observed due to microcrystalline nature).

Composition

Primarily silicon dioxide (SiO2) if silicified (chalcedony/chert), or calcium carbonate (CaCO3) if recrystallized calcite. Often contains trace impurities of iron, manganese, and other elements.

Physical characteristics

• Crystal Habit: Pseudomorph after coral skeletons; cryptocrystalline or microcrystalline aggregates.
• Cleavage Type: None (silicified); perfect rhombohedral (calcitic, but not typically observed).
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy.

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