How to identify River Stone

To correctly identify River Stone (Sedimentary clast, likely chert or basalt), 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 black, grey, white, brown, red, green, or multicolored.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Typically dull to earthy, but can be vitreous (glassy) on fresh surfaces or if composed of quartz/chert, or waxy if chert. Often appears polished due to abrasion.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, often polished surface due to fluvial abrasion. Sub-rounded to well-rounded shapes. Aphanitic (fine-grained) to phaneritic (coarse-grained) depending on the original rock type.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for a clast; the original rock may have had crystalline structure, but the clast itself is a fragment.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for a clast; cleavage would be a property of the constituent minerals within the original rock, not the clast itself.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in active and ancient riverbeds, floodplains, alluvial fans, and terraces. Also present in sedimentary rocks formed from these environments (e.g., conglomerates).

Key Facts

Hardness

Variable, depending on the constituent rock type. Chert (7 on Mohs), Basalt (5-6), Quartzite (7), Granite (6-7).

Specific Gravity

Variable, depending on the constituent rock type. Chert (2.58-2.65), Basalt (2.7-3.3), Quartzite (2.6-2.7), Granite (2.6-2.7).

Crystal System

Not applicable for a rock clast; refers to the crystal structure of individual minerals.

Color

Highly variable, reflecting the source rock.

Luster

Dull to earthy, sometimes waxy or vitreous.

Transparency

Opaque to translucent (e.g., some chert varieties).

Fracture

Variable, depending on the constituent rock type. Chert (conchoidal), Basalt (uneven to conchoidal), Quartzite (conchoidal to granular), Granite (uneven).

Cleavage

Not applicable for a rock clast; refers to the cleavage of individual minerals.

Composition

Variable, depending on the source rock. Can be predominantly silica (chert, quartzite), mafic minerals (basalt), or a mix of quartz, feldspar, micas (granite).

Physical characteristics

• Crystal Habit: Not applicable for a rock clast.
• Cleavage Type: Not applicable for a rock clast.
• Fracture Type: Variable, depending on the constituent rock type (e.g., conchoidal for chert, uneven for basalt).
• Tenacity: Variable, depending on the constituent rock type (e.g., brittle for chert, tough for basalt).
• Luster Type: Dull, earthy, waxy, or vitreous.

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