How to identify River Rock

To correctly identify River Rock (Sedimentary or Metamorphic Rock (e.g., Quartzite, Sandstone, 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 lithology. Can be white, gray, black, brown, red, green, or multicolored.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous, depending on the mineral composition and degree of polish from fluvial action. Often a subdued, earthy luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, often polished surface due to abrasion. Clastic texture, with individual grains or clasts visible. Grain size can vary from sand to boulder.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the rock as a whole; individual mineral grains within the clasts may exhibit crystal forms, but the clasts themselves are typically anhedral and rounded.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for the rock as a whole. Individual mineral components within the clasts may exhibit cleavage (e.g., feldspar, mica), but the overall rock does not have a characteristic 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 preserved in the rock record.

Key Facts

Hardness

Variable, depending on the constituent minerals and rocks. For example, quartz (7), feldspar (6-6.5), chert (6.5-7).

Specific Gravity

Variable, typically ranging from 2.5 to 3.0 g/cm³, depending on the density of the constituent rocks.

Crystal System

Not applicable for a composite rock. Individual mineral grains within the clasts will have their own crystal systems.

Color

Highly diverse, reflecting the source rocks.

Luster

Dull to earthy, sometimes sub-vitreous.

Transparency

Opaque to translucent, depending on the constituent minerals and rock types.

Fracture

Conchoidal (for quartz/chert), irregular, or splintery, depending on the constituent rock type.

Cleavage

Absent as a whole rock property. Present in individual mineral grains within the clasts (e.g., mica, feldspar).

Composition

Polymictic, meaning composed of multiple rock and mineral types. Common constituents include quartz, feldspar, mica, chert, granite, basalt, sandstone, shale, limestone, and various metamorphic rocks.

Physical characteristics

• Crystal Habit: Not applicable for a composite rock; clasts are typically anhedral and rounded.
• Cleavage Type: Absent as a whole rock property.
• Fracture Type: Variable, depending on the constituent rock types (e.g., conchoidal for chert, irregular for granite).
• Tenacity: Brittle, but overall strength depends on the weakest constituent and cementation (if any).
• Luster Type: Dull, earthy, or sub-vitreous.

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