How to identify River Cobble

To correctly identify River Cobble (Sedimentary Rock (likely sandstone or greywacke)), 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. Sandstone cobbles can be white, grey, tan, brown, red, or yellow. Greywacke cobbles are typically dark grey to greenish-grey.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, depending on the mineralogy and surface weathering. Quartz grains within sandstone may exhibit a vitreous luster on fresh breaks.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, detrital. The surface is typically smooth due to abrasion, but can be slightly rough if the rock is porous or has a coarse grain size. Internally, sandstone cobbles will show sand-sized grains (0.0625 mm to 2 mm), while greywacke cobbles will show a mixture of sand-sized grains, rock fragments, and a fine-grained matrix.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for a rock; refers to the overall shape of the clast, which is typically rounded to sub-rounded.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for a rock. Individual mineral grains within the cobble (e.g., feldspar) may exhibit cleavage, but the rock itself does not.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial environments (river beds, floodplains, river terraces), alluvial fans, and sometimes glacial outwash plains where meltwater streams transport and round clasts.

Key Facts

Hardness

Variable, depending on constituent minerals and cementation. For sandstone, typically 6-7 (Mohs) for quartz grains, but the rock's overall hardness can be lower due to cement. Greywacke is generally 5-7 (Mohs).

Specific Gravity

Variable, typically 2.5-2.8 g/cm³ for sandstone and greywacke.

Crystal System

Not applicable for a rock. Constituent minerals (e.g., quartz) are trigonal.

Color

Highly variable, reflecting source rock lithology.

Luster

Dull to earthy.

Transparency

Opaque to translucent (for individual quartz grains).

Fracture

Conchoidal to irregular for individual grains; irregular to blocky for the rock.

Cleavage

Not applicable for a rock. Individual minerals may exhibit cleavage (e.g., feldspar).

Composition

Primarily quartz, feldspar, and rock fragments (lithic fragments) with a matrix of clay, silt, and/or chemical cement (e.g., silica, calcite, iron oxides). Sandstone cobbles are dominated by quartz; greywacke cobbles have significant lithic fragments and a muddy matrix.

Physical characteristics

• Crystal Habit: Not applicable for a rock; refers to the overall rounded to sub-rounded shape of the clast.
• Cleavage Type: Not applicable for a rock.
• Fracture Type: Irregular to blocky for the rock; conchoidal for individual quartz grains.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

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