How to identify River Rock

To correctly identify River Rock (Sedimentary Rock (likely sandstone or quartzite pebble)), 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 pebbles can be white, gray, tan, brown, red, or yellow. Quartzite pebbles are typically white, gray, pink, or reddish-brown, often with a translucent quality.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy for sandstone pebbles; vitreous (glassy) to sugary for quartzite pebbles.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth and rounded due to fluvial abrasion. Sandstone pebbles may feel gritty if individual sand grains are discernible. Quartzite pebbles feel very smooth and hard.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for a rock; individual mineral grains within the rock may exhibit anhedral to subhedral forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for a rock. Individual mineral grains (e.g., feldspar) may show cleavage, but the rock as a whole does not.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial environments (riverbeds, stream channels, floodplains, alluvial fans), lacustrine (lake) environments, and sometimes marine environments where rivers discharge into the sea.

Key Facts

Hardness

Variable, depending on composition. For sandstone pebbles, typically 6-7 (Mohs) if quartz-cemented, but can be lower if poorly cemented. For quartzite pebbles, 7 (Mohs).

Specific Gravity

Variable, typically 2.6-2.7 for quartz-rich varieties.

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 (sandstone); vitreous to sugary (quartzite).

Transparency

Opaque to translucent (especially quartzite).

Fracture

Conchoidal to irregular (quartzite); granular to irregular (sandstone).

Cleavage

Not applicable for a rock. Quartz has no cleavage.

Composition

Predominantly quartz (SiO2) for sandstone and quartzite pebbles, with minor amounts of feldspar, micas, and other accessory minerals, and a cementing agent (silica, calcite, iron oxides, clay) for sandstone.

Physical characteristics

• Crystal Habit: Not applicable for a rock; constituent minerals are typically anhedral to subhedral grains.
• Cleavage Type: None (for quartz); may be present in other constituent minerals like feldspar.
• Fracture Type: Conchoidal to irregular (quartzite); granular to irregular (sandstone).
• Tenacity: Brittle.
• Luster Type: Dull to earthy (sandstone); vitreous to sugary (quartzite).

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