How to identify Crushed Gravel

To correctly identify Crushed Gravel (Mixed Sedimentary and 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

   Highly variable, depending on the constituent rock types. Can range from light grays, whites, and tans (limestone, sandstone) to darker grays, greens, and blacks (shale, schist, gneiss). Often a heterogeneous mix of colors.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, depending on the individual rock fragments. Some metamorphic fragments (e.g., mica in schist) may exhibit a sub-vitreous or pearly luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Angular to sub-angular fragments, rough surfaces, clastic. The texture is defined by the mechanical breakage rather than natural weathering or depositional processes. Grain sizes are typically well-sorted by screening, but the overall aggregate contains a range of sizes.

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 as a whole. Individual rock fragments may exhibit cleavage planes if they are composed of minerals with strong cleavage (e.g., mica, calcite) or if the rock itself has a planar fabric (e.g., slate, schist).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Crushed gravel is an anthropogenic material, not found in a natural geological environment as a 'rock type'. Its source materials are extracted from quarries (bedrock) or gravel pits (unconsolidated sediments) which represent various geological environments (e.g., ancient marine basins for limestone, mountain belts for metamorphic rocks, fluvial or glacial deposits for natural gravels).

Key Facts

Hardness

Variable, dependent on constituent minerals (e.g., quartz 7, calcite 3, feldspar 6). The aggregate itself does not have a single Mohs hardness.

Specific Gravity

Variable, typically ranging from 2.5 to 2.9 g/cm³ for the bulk aggregate, depending on the density of the source rocks.

Crystal System

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

Color

Highly variable, reflecting the diverse lithologies present.

Luster

Dull to earthy, with occasional sub-vitreous or pearly components.

Transparency

Opaque to translucent, depending on the individual rock fragments.

Fracture

Conchoidal, irregular, or splintery, depending on the mineralogy and texture of individual rock fragments.

Cleavage

Absent in the aggregate as a whole; present in specific mineral grains or rock types within the fragments (e.g., mica, calcite, slate).

Composition

A mechanical mixture of various rock fragments, predominantly sedimentary (e.g., limestone, sandstone, shale, chert) and metamorphic (e.g., slate, schist, gneiss, quartzite). Mineralogy is diverse, including quartz, feldspar, calcite, dolomite, micas, amphiboles, pyroxenes, and clay minerals.

Physical characteristics

• Crystal Habit: Not applicable to the aggregate. Individual mineral grains within fragments may show characteristic habits.
• Cleavage Type: Not applicable to the aggregate. Cleavage may be observed in individual mineral grains or rock types within the fragments (e.g., perfect basal in mica, rhombohedral in calcite, slaty in slate).
• Fracture Type: Irregular to conchoidal, depending on the specific rock fragment.
• Tenacity: Brittle for most constituent rock fragments.
• Luster Type: Dull, earthy, sub-vitreous, or pearly.

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