How to identify Fossilized Coral

To correctly identify Fossilized Coral (Rugose Coral (horn coral) or Tabulate Coral), 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, black, white, pink, red, yellow, and blue. Often exhibits concentric banding or mottled patterns.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy, especially when agatized. Can be dull if not polished or if the fossilization is less complete.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular when polished. Unpolished surfaces may feel rough. The key identifying feature is the preserved internal structure of the coral, such as septa, tabulae, or corallites, which can be seen on cut or weathered surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (chalcedony/agate) or microcrystalline (calcite). The overall form reflects the original coral colony or individual polyp structure (e.g., horn-shaped for solitary rugose corals, massive or branching for colonial forms).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its cryptocrystalline nature (if agatized). If preserved as calcite, it may exhibit rhombohedral cleavage, but this is less common for well-preserved fossils.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in marine sedimentary rocks, particularly limestones, shales, and sandstones that formed in ancient shallow, warm, clear marine environments conducive to coral growth. Often associated with ancient reef complexes or carbonate platforms.

Key Facts

Hardness

6.5-7 on Mohs scale (if agatized/silicified); 3-4 (if calcitic).

Specific Gravity

2.58-2.64 (if agatized/silicified); 2.71 (if calcitic).

Crystal System

Trigonal (for calcite replacement); Cryptocrystalline (for chalcedony/agate replacement).

Color

Highly variable, often exhibiting original coral patterns.

Luster

Vitreous to waxy (agatized), dull (calcitic).

Transparency

Opaque to translucent.

Fracture

Conchoidal (if agatized); Uneven (if calcitic).

Cleavage

None (if agatized); Rhombohedral (if calcitic, but rarely observed in fossil form).

Composition

Primarily SiO2 (silica) if agatized; CaCO3 (calcium carbonate) if preserved as calcite.

Physical characteristics

• Crystal Habit: Pseudomorphic after original coral structure; cryptocrystalline aggregates.
• Cleavage Type: Absent (agatized); Perfect rhombohedral (calcitic, but rare).
• Fracture Type: Conchoidal (agatized); Uneven to subconchoidal (calcitic).
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy.

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