How to identify Ammonite fossil

To correctly identify Ammonite fossil (Ammonoidea), 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 original shell composition and the mineral replacement during fossilization. Common colors include shades of gray, brown, black, white, yellow, red, and iridescent hues (e.g., Ammolite).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable, from dull to vitreous, pearly, or metallic (pyritized specimens). Iridescent specimens exhibit an opalescent or nacreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically smooth to slightly rough, reflecting the original shell surface or the texture of the replacing mineral. Internal molds may show suture patterns.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not a mineral crystal form, but rather the preserved biological structure of the ammonite shell. Typically a planispiral (flat-coiled) or helicospiral (conical-coiled) form, with internal septa and complex suture lines.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the fossil itself, as it is a biological structure. However, the mineral infilling or replacement may exhibit cleavage (e.g., calcite has perfect rhombohedral cleavage).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found predominantly in marine sedimentary rocks, including shales, limestones, marls, sandstones, and mudstones. These rocks formed in ancient shallow to deep marine environments, often in epicontinental seas or continental shelf settings.

Key Facts

Hardness

Variable, depending on the mineral composition of the fossil. Original aragonite is 3.5-4 on Mohs scale. Calcite replacement is 3. Pyrite replacement is 6-6.5. Silicified specimens are 7.

Specific Gravity

Variable, depending on the mineral composition. Aragonite is 2.95. Calcite is 2.71. Pyrite is 4.95-5.10. Silica (quartz) is 2.65.

Crystal System

Not applicable to the fossil as a biological structure. The original shell was biogenic aragonite (orthorhombic). Replacing minerals will have their own crystal systems (e.g., calcite is trigonal, pyrite is isometric, quartz is trigonal).

Color

Highly variable, as described above.

Luster

Highly variable, as described above.

Transparency

Opaque to translucent, depending on the preservation and mineral replacement.

Fracture

Not applicable to the fossil as a whole. The replacing mineral may exhibit conchoidal (silica), uneven (calcite), or irregular fracture.

Cleavage

Not applicable to the fossil as a whole. The replacing mineral may exhibit cleavage (e.g., perfect rhombohedral for calcite).

Composition

Primarily calcium carbonate (CaCO3) in the original shell (aragonite), often replaced by calcite (CaCO3), pyrite (FeS2), silica (SiO2), or iron oxides during fossilization.

Physical characteristics

• Crystal Habit: Not applicable; refers to the preserved biological form of the ammonite shell (coiled, chambered).
• Cleavage Type: Not applicable to the fossil itself; depends on the replacing mineral.
• Fracture Type: Not applicable to the fossil itself; depends on the replacing mineral.
• Tenacity: Brittle, similar to the host rock or the replacing mineral.
• Luster Type: Variable, as described above.

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