How to identify Crushed Gravel

To correctly identify Crushed Gravel (Mixed Sedimentary/Metamorphic 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

   Extremely variable, dependent on source rock. Can be grey, white, black, brown, red, green, or mixed.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous, dependent on the mineralogy of the constituent rock fragments. Freshly broken surfaces may appear dull to earthy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Angular to sub-angular fragments with rough, freshly fractured surfaces. Grain size is typically well-sorted by screening, but overall texture is clastic and fragmental.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the aggregate as a whole. Individual mineral grains within the rock fragments may exhibit their characteristic crystal forms, but the fragments themselves are irregular.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the aggregate. Cleavage may be present within individual mineral grains or rock fragments (e.g., mica in schist fragments, calcite in limestone fragments).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Anthropogenic. Found in quarries, construction sites, roadbeds, and stockpiles. The source rocks originate from various natural geological environments (e.g., marine basins for limestone, mountain belts for metamorphic rocks, volcanic regions for igneous rocks).

Key Facts

Hardness

Variable, dependent on the hardness of the constituent rock fragments (e.g., quartz-rich fragments are harder than calcite-rich fragments). Mohs hardness can range from 3 (limestone) to 7 (quartzite).

Specific Gravity

Variable, typically 2.5 to 2.9 g/cm³ for common rock types, but can range from 2.0 to 3.3 g/cm³ depending on the density of the source rock.

Crystal System

Not applicable to the aggregate. Individual mineral grains within the fragments will have their own crystal systems.

Color

Highly variable, reflecting the source rock geology.

Luster

Dull to vitreous, depending on the mineralogy and freshness of fracture surfaces.

Transparency

Opaque to translucent, depending on the individual rock fragments.

Fracture

Conchoidal, irregular, or splintery, depending on the constituent rock type and mineralogy.

Cleavage

Not applicable to the aggregate. Cleavage may be present in individual mineral grains or rock fragments.

Composition

Mixed lithic fragments (e.g., quartz, feldspar, calcite, dolomite, mica, amphibole, pyroxene, clay minerals) derived from various igneous, sedimentary, and metamorphic rocks.

Physical characteristics

• Crystal Habit: Not applicable to the aggregate; fragments are irregular.
• Cleavage Type: Not applicable to the aggregate; present in constituent minerals/rocks.
• Fracture Type: Variable (conchoidal, irregular, splintery) depending on the rock type.
• Tenacity: Brittle (fragments break easily under impact).
• Luster Type: Variable (dull, earthy, vitreous, greasy) depending on the constituent minerals.

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