How to identify Crushed Gravel

To correctly identify Crushed Gravel (Mixed Sedimentary/Igneous/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 source rock. Can be grey, black, white, red, brown, tan, or multicolored.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous, depending on the mineralogy of the constituent fragments. Often appears dull due to fractured surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Angular to sub-angular, rough, and gritty due to fractured surfaces. Fragments are typically well-sorted by size during processing.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the aggregate itself, but individual mineral grains within the rock fragments may exhibit their characteristic crystal forms (e.g., quartz, feldspar).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the aggregate as a whole. Individual mineral grains within the rock fragments may exhibit cleavage (e.g., feldspar, mica).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Crushed gravel is an anthropogenic material, not naturally occurring in its crushed form. Its source rocks originate from various geological environments, including igneous intrusions (granite, gabbro), volcanic flows (basalt), sedimentary basins (limestone, sandstone), and metamorphic terrains (gneiss, quartzite).

Key Facts

Hardness

Variable, depending on constituent rock types (e.g., quartz 7, calcite 3). Overall bulk hardness is a function of the weakest component and inter-particle friction.

Specific Gravity

Variable, typically 2.5 to 2.9 g/cm³ (e.g., limestone ~2.7, granite ~2.65, basalt ~2.9).

Crystal System

Not applicable (aggregate of various rock fragments).

Color

Highly variable, reflecting source rock.

Luster

Dull to vitreous, depending on mineralogy.

Transparency

Opaque to translucent, depending on individual fragments.

Fracture

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

Cleavage

Not applicable to the aggregate; present in some constituent minerals.

Composition

Mixed silicates (quartz, feldspar, mica, amphibole, pyroxene), carbonates (calcite, dolomite), oxides (iron oxides), etc., depending on the source rock types.

Physical characteristics

• Crystal Habit: Not applicable (aggregate).
• Cleavage Type: Not applicable to the aggregate; present in some constituent minerals (e.g., perfect in mica, good in feldspar).
• Fracture Type: Irregular to conchoidal, depending on the rock type and mineral composition of individual fragments.
• Tenacity: Brittle (individual fragments).
• Luster Type: Variable (e.g., earthy, dull, vitreous).

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