How to identify River Rock

To correctly identify River Rock (Sedimentary Rock (likely sandstone or quartzite, rounded by fluvial transport)), 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, depending on the source rock. Common colors include shades of gray, brown, tan, red, white, black, and often mixed or mottled.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes vitreous if individual mineral grains (like quartz) are exposed and polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, rounded to sub-rounded surfaces due to fluvial abrasion. Can range from fine-grained (sandstone clasts) to coarse-grained (granite clasts).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the rock as a whole; individual mineral grains within the clasts may exhibit crystalline forms, but the clasts themselves are irregular, rounded fragments.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the rock as a whole. Individual mineral components within the clasts (e.g., feldspar in granite clasts) may exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial environments, including riverbeds, floodplains, terraces, and alluvial fans. Also found in ancient river deposits that have been uplifted and exposed.

Key Facts

Hardness

Variable, depends on the constituent minerals. For quartzite clasts, Mohs hardness is 7. For sandstone clasts, it can range from 6-7 (quartz grains) but the rock as a whole may be less cohesive.

Specific Gravity

Variable, typically 2.6-2.8 g/cm³ for common silicate rocks like sandstone or quartzite.

Crystal System

Not applicable to the rock as a whole; refers to individual mineral components.

Color

Highly variable, reflecting source rock lithology.

Luster

Dull to earthy, sometimes vitreous.

Transparency

Opaque to translucent (for individual quartz grains).

Fracture

Conchoidal (for quartz-rich clasts) to irregular, depending on the rock type.

Cleavage

Not applicable to the rock as a whole; refers to individual mineral components.

Composition

Clastic sedimentary rock, composed of various rock fragments and mineral grains (e.g., quartz, feldspar, mica, lithic fragments) cemented together or unconsolidated. Predominantly silicate minerals.

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole; clasts are rounded fragments.
• Cleavage Type: Not applicable to the rock as a whole.
• Fracture Type: Variable, depending on the clast composition (e.g., conchoidal for quartz, irregular for other rock types).
• Tenacity: Brittle.
• Luster Type: Dull to earthy, sometimes vitreous.

Explore River Rock