How to identify River Rock

To correctly identify River Rock (Sedimentary clast (likely chert or fine-grained igneous rock)), 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. 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 mineral composition and surface polish. Often appears dull when dry, but can be somewhat lustrous when wet.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, often polished surface due to abrasion. Sub-rounded to well-rounded shape, lacking sharp edges or angularity. Can feel gritty if composed of coarser grains or if the surface is weathered.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable, as river rock is a clast (fragment) of pre-existing rock, not a single crystal or mineral. Individual mineral grains within the clast may exhibit their characteristic crystal forms if visible.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the clast as a whole. Cleavage may be present within the constituent minerals of the clast (e.g., feldspar in granite clasts), but the clast itself does not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial environments (rivers, streams, floodplains), alluvial fans, and ancient river deposits. They are found wherever water flow has transported and abraded rock fragments.

Key Facts

Hardness

Variable, depending on composition. Chert: 6.5-7 (Mohs); Quartzite: 7 (Mohs); Basalt: 6-7 (Mohs).

Specific Gravity

Variable, depending on composition. Chert: 2.58-2.64; Quartzite: 2.6-2.7; Basalt: 2.8-3.0.

Crystal System

Not applicable to the clast as a whole. Constituent minerals will have their own crystal systems (e.g., quartz is trigonal, feldspar is triclinic/monoclinic).

Color

Highly variable, reflecting source rock lithology.

Luster

Dull to vitreous.

Transparency

Opaque to translucent, depending on composition and thickness.

Fracture

Conchoidal (if chert or fine-grained igneous), irregular, or granular, depending on the constituent minerals and rock type.

Cleavage

Absent in the clast as a whole. May be present in individual mineral grains within the clast.

Composition

Predominantly silica (SiO2) in the form of chert or quartzite, or various silicate minerals (e.g., feldspar, pyroxene, amphibole) in fine-grained igneous rocks. Can also include fragments of other durable rock types.

Physical characteristics

• Crystal Habit: Not applicable to the clast. Individual mineral grains within the clast may exhibit anhedral to subhedral forms.
• Cleavage Type: Absent in the clast. If present in constituent minerals, it would be specific to that mineral (e.g., perfect in feldspar, prismatic in pyroxene).
• Fracture Type: Conchoidal (for chert/fine-grained igneous), irregular, or granular.
• Tenacity: Brittle.
• Luster Type: Dull to vitreous.

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