How to identify Fossilized Rugose Coral

To correctly identify Fossilized Rugose Coral (Rugosa (extinct order of corals)), 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 mineralizing agent and host rock. Common colors include shades of gray, brown, black, white, cream, tan, and sometimes reddish or yellowish hues due to iron oxides.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Typically dull to vitreous, depending on the degree of silicification or calcification and the preservation quality. Silicified specimens can exhibit a waxy or glassy luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can range from smooth to rough, often preserving the external growth lines and septal grooves. Internally, polished specimens reveal the characteristic septal patterns, tabulae, and dissepiments.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not a true crystal form in the mineralogical sense, but rather a biogenic structure. The fossil preserves the original coral morphology, which can be solitary (conical, horn-shaped) or colonial (massive, branching, columnar).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No inherent cleavage as a fossilized organism. However, the mineral infilling or replacement (e.g., calcite) may exhibit its characteristic cleavage (rhombohedral for calcite).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in marine sedimentary rocks, particularly limestones, shales, and sandstones, that formed in shallow, warm, clear marine environments, indicative of ancient reef or shelf settings. Often associated with other marine fossils like brachiopods, crinoids, and trilobites.

Key Facts

Hardness

3-7 on Mohs scale, depending on the mineral composition of the fossil (calcite is 3, quartz/silica is 7).

Specific Gravity

2.6-2.8 for calcified specimens; 2.65 for silicified specimens.

Crystal System

Not applicable to the fossilized organism itself. The infilling/replacement minerals will have their own crystal system (e.g., trigonal for calcite, hexagonal for quartz).

Color

Highly variable, influenced by the mineralizing agent and impurities.

Luster

Dull, earthy, waxy, or vitreous.

Transparency

Opaque to translucent, depending on preservation and mineral composition.

Fracture

Conchoidal (if silicified) to uneven/splintery (if calcified or poorly preserved).

Cleavage

None inherent to the fossil structure; may exhibit cleavage of replacement minerals (e.g., rhombohedral for calcite).

Composition

Primarily calcium carbonate (CaCO3) in the form of calcite, or silicon dioxide (SiO2) in the form of quartz/chalcedony, replacing the original aragonite/calcite skeleton. Minor impurities can include iron oxides, clays, or organic matter.

Physical characteristics

• Crystal Habit: Not a mineral crystal habit, but rather a biogenic morphology: solitary (horn-shaped, conical) or colonial (massive, fasciculate, phaceloid).
• Cleavage Type: Absent in the fossil structure itself; cleavage may be present in the replacement mineral (e.g., perfect rhombohedral for calcite).
• Fracture Type: Conchoidal (silicified) to uneven/splintery (calcified).
• Tenacity: Brittle.
• Luster Type: Dull, earthy, waxy, or vitreous.

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