How to identify Ammonite Fossil

To correctly identify Ammonite Fossil (Ammonoidea (fossil)), 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 shades of brown, grey, 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 (glassy) or pearly, depending on the preservation and mineral composition. Pyritized ammonites can have a metallic luster.

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 structure, if exposed, may show septa and suture lines. The surrounding rock matrix can be fine-grained (shale, limestone) or coarser (sandstone).

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 biological form of the ammonite shell, typically a planispiral coil. Internal structures may show mineral infillings with crystalline habits (e.g., calcite crystals).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the fossilized shell itself, as it is an organic structure replaced by minerals. The minerals infilling or replacing the shell may exhibit cleavage (e.g., calcite has perfect rhombohedral cleavage).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine sedimentary rocks, particularly shales, limestones, marls, and sandstones, formed in ancient shallow to deep marine environments. Often found in association with other marine fossils.

Key Facts

Hardness

Variable, depending on the mineral composition of the fossil. Calcite-replaced ammonites are typically 3 on the Mohs scale. Pyritized ammonites are 6-6.5. Silicified ammonites are 7.

Specific Gravity

Variable, depending on the mineral composition. Calcite-replaced: ~2.7. Pyritized: ~5.0. Silicified: ~2.65.

Crystal System

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

Color

Highly variable, influenced by the replacing minerals and matrix. Common colors include browns, greys, blacks, whites, yellows, reds, and iridescent (e.g., ammolite).

Luster

Variable, from dull to vitreous, pearly, or metallic, depending on preservation and mineralogy.

Transparency

Opaque to translucent, depending on the mineral composition and thickness.

Fracture

Variable, depending on the mineral composition. Calcite-replaced may show conchoidal to uneven fracture. Pyritized may show uneven to subconchoidal. Silicified shows conchoidal fracture.

Cleavage

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

Composition

Primarily calcium carbonate (CaCO3) in the original shell, often replaced by calcite, aragonite, pyrite (FeS2), silica (SiO2), or other minerals during fossilization. The surrounding matrix is typically sedimentary rock.

Physical characteristics

• Crystal Habit: Not applicable to the fossil itself. The fossil retains the biogenic form of the ammonite shell, typically a planispiral coil. Infilling minerals may show their characteristic crystal habits.
• Cleavage Type: Not applicable to the fossilized shell. Infilling minerals may exhibit cleavage (e.g., calcite: perfect rhombohedral).
• Fracture Type: Variable, depending on the replacing mineral (e.g., conchoidal for silicified, uneven for pyritized).
• Tenacity: Brittle, varying with the mineral composition.
• Luster Type: Variable (dull, vitreous, pearly, metallic).

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