How to identify Ammonite Fossil

To correctly identify Ammonite Fossil (Ammonoidea (subclass)), 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 mineral composition of the fossil and the surrounding matrix. Common colors include grey, brown, black, white, yellow, red, and iridescent (e.g., ammolite).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable, from dull to vitreous, pearly, or even iridescent (in the case of ammolite, which is aragonite).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to ribbed or ornamented on the external surface of the shell. The internal cast may show suture lines.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not a mineral with a crystal form, but the fossil itself retains the original shell morphology, typically a planispiral coil. Internal infillings can be crystalline (e.g., calcite, pyrite).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the fossilized shell itself. However, the infilling minerals may exhibit cleavage (e.g., calcite has perfect rhombohedral cleavage).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in marine sedimentary rocks, particularly shales, limestones, marls, and sandstones, which formed in ancient shallow to deep marine environments. Often associated with other marine fossils.

Key Facts

Hardness

Variable, depending on the mineral composition of the fossil (e.g., calcite 3, pyrite 6-6.5, silica 7). The original shell was aragonite (3.5-4).

Specific Gravity

Variable, depending on the mineral composition (e.g., calcite 2.7, pyrite 5.0, silica 2.65).

Crystal System

Not applicable to the fossilized shell itself. Infilling minerals will have their own crystal systems (e.g., calcite is trigonal, pyrite is isometric).

Color

Highly variable, as described above.

Luster

Variable, as described above.

Transparency

Opaque to translucent, depending on the infilling material and preservation.

Fracture

Not applicable to the fossilized shell itself. Infilling minerals may exhibit conchoidal (silica) or uneven (calcite, pyrite) fracture.

Cleavage

Not applicable to the fossilized shell itself. Infilling minerals may exhibit cleavage (e.g., calcite has perfect rhombohedral cleavage).

Composition

Primarily calcium carbonate (calcite or aragonite) if the original shell material is preserved or recrystallized, or replaced by other minerals such as pyrite (iron sulfide), silica (quartz), or phosphate. The original shell was composed of aragonite (CaCO3).

Physical characteristics

• Crystal Habit: Not applicable to the fossil. The fossil retains the biogenic form of the ammonite shell, typically a planispiral coil.
• Cleavage Type: Not applicable to the fossil. Cleavage would be observed in the mineral infilling, if present (e.g., rhombohedral for calcite).
• Fracture Type: Not applicable to the fossil. Fracture would be observed in the mineral infilling (e.g., conchoidal for quartz, uneven for pyrite).
• Tenacity: Brittle, depending on the mineral composition and degree of fossilization.
• Luster Type: Variable, as described above.

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