How to identify Gravel/Pebbles (Mixed Sedimentary Rocks)

To correctly identify Gravel/Pebbles (Mixed Sedimentary Rocks) (Mixed clastic sedimentary rocks (e.g., chert, quartz, basalt, granite 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 fragments. Can be shades of grey, brown, red, black, white, or multicolored.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable, depending on the individual clasts. Can range from dull to vitreous, earthy, or greasy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, detrital. Consists of individual rock fragments (clasts) ranging from 2 mm to over 256 mm in diameter. Typically rounded to sub-rounded, but can be sub-angular to angular if transport distance was short or energy was low. Unconsolidated.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable, as it is an aggregate of rock fragments, not a single mineral with a defined crystal form. Individual mineral grains within the clasts may exhibit crystal forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the aggregate. Individual mineral grains within the clasts may exhibit cleavage (e.g., feldspar, mica).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial (riverbeds, floodplains, alluvial fans), marine (beaches, shallow marine shelves), lacustrine (lake shores), glacial (moraines, outwash plains), colluvial (base of slopes).

Key Facts

Hardness

Variable, depending on the hardest constituent clasts (e.g., quartz is 7 on Mohs scale, chert 6.5-7, basalt 6, granite 6-7).

Specific Gravity

Variable, typically ranging from 2.5 to 3.0 g/cm³, depending on the density of the constituent rock types.

Crystal System

Not applicable (aggregate of various rock fragments).

Color

Polychromatic, reflecting the diverse lithologies of the source rocks.

Luster

Variable, from dull to vitreous, depending on the clast composition.

Transparency

Opaque to translucent, depending on individual clasts.

Fracture

Variable, depending on the constituent clasts (e.g., conchoidal for quartz/chert, irregular for basalt/granite).

Cleavage

Not applicable to the aggregate. Present in some constituent minerals (e.g., feldspar, mica).

Composition

Heterogeneous mixture of rock fragments (e.g., chert, quartz, basalt, granite, sandstone, limestone, schist, gneiss) and mineral grains (e.g., quartz, feldspar, mica, amphibole, pyroxene).

Physical characteristics

• Crystal Habit: Not applicable (aggregate).
• Cleavage Type: Not applicable to the aggregate. Cleavage may be present in individual mineral grains within the clasts.
• Fracture Type: Variable, depending on the constituent clasts. Can be conchoidal (quartz, chert), irregular (basalt, granite), or splintery.
• Tenacity: Brittle (for individual clasts). The aggregate is unconsolidated and friable.
• Luster Type: Variable, ranging from dull to vitreous, earthy, or greasy, depending on the dominant clast types.

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