How to identify River Rock

To correctly identify River Rock (Sedimentary or Metamorphic Rock (highly weathered and rounded)), 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, ranging from white, grey, black, brown, red, green, to multicolored, depending on the constituent rock types.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous, depending on the mineralogy of the individual clasts. Often appears dull due to surface weathering and fine sediment coating.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, rounded to sub-rounded surfaces due to fluvial abrasion. Can feel gritty if composed of sandstone or rough if composed of less resistant, weathered rock types. Non-porous to slightly porous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable, as river rock is composed of fragments of pre-existing rocks, not individual crystals. Individual clasts may show relict crystal forms if they are igneous or metamorphic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the rock as a whole. Individual clasts may exhibit cleavage planes if they are composed of minerals like feldspar or mica.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial environments (rivers, streams), alluvial fans, floodplains, and ancient river terraces. Also found in glaciofluvial deposits where meltwater streams have reworked glacial sediments.

Key Facts

Hardness

Variable, depending on the constituent rock types (e.g., quartz clasts are 7 on Mohs, limestone clasts are 3-4).

Specific Gravity

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

Crystal System

Not applicable (rock fragments, not individual crystals).

Color

Highly variable, reflecting the source geology.

Luster

Dull to vitreous.

Transparency

Opaque to translucent, depending on the individual clasts.

Fracture

Conchoidal, irregular, or splintery, depending on the individual clasts.

Cleavage

Not applicable to the rock as a whole; present in individual mineral grains within clasts.

Composition

Polymictic, composed of fragments of various igneous, sedimentary, and metamorphic rocks (e.g., granite, basalt, sandstone, limestone, schist, gneiss, quartzite).

Physical characteristics

• Crystal Habit: Not applicable (fragments of pre-existing rocks).
• Cleavage Type: Not applicable to the rock; individual mineral grains may exhibit cleavage.
• Fracture Type: Variable, depending on the constituent rock types.
• Tenacity: Brittle to tough, depending on the constituent rock types.
• Luster Type: Dull, earthy, or vitreous.

Explore River Rock