How to identify Gravel

To correctly identify Gravel (Mixed Sedimentary Grains), 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 constituent rock and mineral fragments. Can be white, gray, black, brown, red, yellow, or a mix of colors.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable, depending on the constituent minerals and rock fragments. Can range from dull to vitreous, earthy, or greasy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, granular. Individual grains are visible and feel gritty. Grain size ranges from 2 mm to 64 mm. Can be angular to well-rounded.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the aggregate as a whole. Individual mineral grains within the gravel may exhibit their characteristic crystal forms, but most are anhedral due to transport and abrasion.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for the aggregate. Individual mineral grains may exhibit cleavage (e.g., feldspar, mica).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial (riverbeds, floodplains, alluvial fans), glacial (moraines, outwash plains), coastal (beaches, marine terraces), lacustrine (lake shores), and colluvial (talus slopes, scree).

Key Facts

Hardness

Variable (depends on constituent minerals/rocks, e.g., quartz is 7 on Mohs scale)

Specific Gravity

Variable (typically 2.5 - 2.8 for common rock types)

Crystal System

Not applicable (aggregate)

Color

Highly variable, reflecting source rocks

Luster

Variable, depending on constituent minerals

Transparency

Opaque to translucent (for individual grains)

Fracture

Variable (conchoidal for quartz, irregular for many rock fragments)

Cleavage

Not applicable for the aggregate; present in some constituent minerals (e.g., feldspar)

Composition

Mixed rock fragments and mineral grains (e.g., quartz, feldspar, chert, granite, basalt, limestone)

Physical characteristics

• Crystal Habit: Not applicable for the aggregate; individual grains are typically anhedral to subhedral.
• Cleavage Type: Not applicable for the aggregate; individual mineral grains may exhibit perfect, good, or poor cleavage.
• Fracture Type: Variable, depending on the constituent materials (e.g., conchoidal for quartz, irregular for many rock fragments).
• Tenacity: Brittle (for individual grains and rock fragments).
• Luster Type: Variable, ranging from dull, earthy, or resinous to vitreous, depending on the dominant mineralogy.

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