How to identify Gravel

To correctly identify Gravel (Mixed sedimentary and igneous rock fragments), 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 these colors.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, unconsolidated. Individual grains are typically rounded to sub-angular, with a rough to smooth surface depending on the degree of abrasion. Grain size ranges from 2 mm to 64 mm.

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 (e.g., quartz often anhedral, feldspar subhedral).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for the aggregate as a whole. 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 (outwash plains, moraines), coastal (beaches, marine terraces), lacustrine (lake shores), and colluvial (talus slopes).

Key Facts

Hardness

Variable (depends on constituent minerals, e.g., quartz 7, feldspar 6-6.5)

Specific Gravity

Variable (typically 2.5-2.8 for common rock types)

Crystal System

Not applicable (aggregate of various rock and mineral fragments)

Color

Highly variable, reflecting the diverse lithologies present.

Luster

Variable, from dull to vitreous, depending on the dominant mineralogy.

Transparency

Opaque to translucent for individual fragments, depending on their composition.

Fracture

Variable, conchoidal for quartz, irregular for many rock fragments.

Cleavage

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

Composition

Mixed fragments of igneous, metamorphic, and sedimentary rocks, along with individual mineral grains (e.g., quartz, feldspar, mica, calcite).

Physical characteristics

• Crystal Habit: Not applicable for the aggregate; individual mineral grains exhibit their characteristic habits.
• Cleavage Type: Not applicable for the aggregate; present in some constituent minerals.
• Fracture Type: Variable, depending on the lithology of individual fragments (e.g., conchoidal for quartz, irregular for many rock fragments).
• Tenacity: Brittle for individual fragments.
• Luster Type: Variable, ranging from dull to vitreous, earthy, or greasy.

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