How to identify River Rock

To correctly identify River Rock (Sedimentary or Metamorphic Rock (weathered)), 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

   Extremely variable, reflecting the source rock. Can be white, grey, black, brown, red, green, or multicolored.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Typically dull to earthy, but can be vitreous or waxy if the source rock has such a luster and the surface is polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, often polished, with a sub-rounded to well-rounded shape due to fluvial abrasion. Absence of sharp edges or angularity.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable, as river rock refers to clasts of pre-existing rocks, not individual crystals. The internal crystal form would depend on the original rock type.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the river rock as a whole; cleavage would be present within the constituent minerals of the original rock type, but not typically observable on the weathered surface.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial environments: riverbeds, stream channels, floodplains, alluvial fans, and terraces. Also found in ancient fluvial deposits that have been uplifted and exposed.

Key Facts

Hardness

Variable, depending on the source rock (e.g., quartz-rich rocks will be harder, limestone softer).

Specific Gravity

Variable, depending on the source rock (e.g., granite ~2.6-2.8 g/cm³, basalt ~2.8-3.0 g/cm³).

Crystal System

Not applicable, as river rock is a clast of a pre-existing rock.

Color

Highly variable, reflecting the source rock.

Luster

Dull to earthy, sometimes vitreous or waxy.

Transparency

Opaque to translucent, depending on the source rock.

Fracture

Not applicable to the whole rock; internal fracture depends on the source rock.

Cleavage

Not applicable to the whole rock; internal cleavage depends on the source rock's minerals.

Composition

Highly variable, composed of fragments of igneous, metamorphic, or sedimentary rocks (e.g., quartz, feldspar, mica, calcite, clay minerals, rock fragments).

Physical characteristics

• Crystal Habit: Not applicable; refers to the overall shape of the clast, which is typically sub-rounded to well-rounded.
• Cleavage Type: Not applicable to the whole rock; depends on the constituent minerals of the source rock.
• Fracture Type: Not applicable to the whole rock; depends on the source rock.
• Tenacity: Variable, depending on the source rock (e.g., brittle for most igneous/metamorphic rocks, sometimes friable for poorly cemented sedimentary rocks).
• Luster Type: Dull, earthy, sometimes vitreous or waxy.

Explore River Rock