How to identify River Rock

To correctly identify River Rock (Sedimentary or Metamorphic Rock (e.g., Basalt, Andesite, Slate, Chert)), 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

   Dull to vitreous, depending on the mineralogy of the individual clasts and whether they are wet or dry.

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 freshly broken.

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 typically anhedral and rounded.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

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 sedimentary deposits that represent fossil river systems.

Key Facts

Hardness

Variable, depends on the hardness of the constituent rock types (e.g., quartz-rich clasts will be 7 on Mohs scale, limestone clasts 3).

Specific Gravity

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

Crystal System

Not applicable to the rock as a whole; individual mineral grains within the clasts will have their own crystal systems.

Color

Polymictic (many colors), reflecting diverse source rocks.

Luster

Dull to vitreous, often smooth and somewhat reflective when wet.

Transparency

Opaque to translucent, depending on the constituent clasts.

Fracture

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

Cleavage

Not applicable to the rock as a whole; individual mineral grains within the clasts may exhibit cleavage.

Composition

Highly variable, composed of fragments of various igneous, metamorphic, and sedimentary rocks and minerals (e.g., quartz, feldspar, mica, chert, basalt, granite, slate, sandstone, limestone).

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole; clasts are typically anhedral and rounded.
• Cleavage Type: Not applicable to the rock as a whole.
• Fracture Type: Variable, depending on the constituent clasts.
• Tenacity: Variable, depending on the constituent clasts.
• Luster Type: Dull to vitreous.

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