How to identify River Pebbles

To correctly identify River Pebbles (Sedimentary/Metamorphic/Igneous Rock Fragments), 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 constituent rock types. Can be white, black, grey, brown, red, green, or multicolored.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable, typically dull to vitreous, depending on the lithology. Quartz pebbles may have a vitreous luster, while basalt pebbles might be dull.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly rough, often polished due to abrasion. The surface may show percussion marks or minor pitting.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the aggregate as a whole. Individual clasts may exhibit relict crystal forms if they are crystalline rocks (e.g., granite, basalt).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the aggregate. Individual clasts may exhibit cleavage if they are composed of minerals with good cleavage (e.g., feldspar in granite, calcite in limestone).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial environments, including riverbeds, riverbanks, floodplains, and alluvial fans. Also found in ancient fluvial deposits that have been lithified into conglomerates.

Key Facts

Hardness

Variable, depending on the constituent lithologies (e.g., quartz ~7, limestone ~3).

Specific Gravity

Variable, typically ranging from 2.5 to 3.0 g/cm³ depending on the dominant lithologies.

Crystal System

Not applicable to the aggregate; individual clasts may be crystalline or amorphous.

Color

Highly variable, reflecting the diverse source rocks.

Luster

Variable, from dull to vitreous.

Transparency

Variable, from opaque to translucent (e.g., quartz pebbles).

Fracture

Variable, depending on the lithology (e.g., conchoidal for quartz, irregular for many others).

Cleavage

Not applicable to the aggregate; individual clasts may or may not exhibit cleavage.

Composition

Mixed, consisting of fragments of various igneous, metamorphic, and sedimentary rocks and minerals (e.g., quartz, feldspar, mica, calcite, rock fragments).

Physical characteristics

• Crystal Habit: Not applicable to the aggregate; individual clasts retain the habit of their source rock/mineral.
• Cleavage Type: Not applicable to the aggregate; depends on individual clasts.
• Fracture Type: Variable, depending on the lithology of individual clasts.
• Tenacity: Variable, depending on the lithology of individual clasts.
• Luster Type: Variable, from dull to vitreous, depending on the lithology.

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