How to identify Gravel

To correctly identify Gravel (Crushed aggregate), 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, depends on the source rock. Can be white, grey, black, brown, red, yellow, or mixed colors.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous, depending on the constituent minerals and the presence of weathering or dust.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Natural gravel is typically rounded to sub-rounded due to abrasion during transport. Crushed aggregate is angular with sharp edges and fractured surfaces. The surface can be rough or smooth depending on the mineralogy and degree of weathering.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual grains may exhibit crystal forms if they are single mineral crystals (e.g., quartz), but the aggregate itself does not have a defined crystal form.

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 aggregate may exhibit cleavage (e.g., feldspar, calcite).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Natural gravel is found in fluvial (riverbeds, floodplains), glacial (moraines, outwash plains), lacustrine (lake shores), and marine (beaches, continental shelves) environments. Crushed aggregate is produced from quarries extracting bedrock (e.g., limestone, granite, basalt, sandstone) or from recycled concrete/asphalt.

Key Facts

Hardness

Variable, depends on constituent minerals (e.g., Quartz 7, Calcite 3)

Specific Gravity

Variable, typically 2.5 - 2.9 g/cm³ (depends on constituent minerals)

Crystal System

Not applicable (aggregate)

Color

Highly variable

Luster

Dull to vitreous

Transparency

Opaque to translucent (individual grains can be transparent)

Fracture

Conchoidal, irregular, or splintery (depends on constituent minerals)

Cleavage

Not applicable to the aggregate; individual minerals may exhibit cleavage

Composition

Variable, commonly quartz, feldspar, calcite, dolomite, chert, granite, basalt, gneiss, etc.

Physical characteristics

• Crystal Habit: Not applicable (aggregate)
• Cleavage Type: Not applicable to the aggregate; individual minerals may show perfect, good, or poor cleavage
• Fracture Type: Variable (depends on constituent minerals)
• Tenacity: Brittle (individual grains)
• Luster Type: Variable (e.g., vitreous for quartz, dull for weathered surfaces)

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