How to identify Weathered Pebble

To correctly identify Weathered Pebble (Sedimentary rock (likely sandstone or quartzite) with biological growth), 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

   Highly variable, depending on the parent rock and the biological growth. Parent rock colors can range from white, grey, tan, brown, red (for sandstone/quartzite) to black. Biological growth adds patches of green, grey-green, yellow, orange, black, or white. Lichens often form crustose, foliose, or fruticose patterns.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy on the weathered surface and biological growth. The underlying rock, if exposed, might show a vitreous (quartz) or granular luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rough, granular, or slightly pitted on the weathered rock surface. The biological growth can be crusty, powdery, leafy, or filamentous, adding a distinct texture that is often slightly raised or adherent to the rock.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the pebble as a whole, nor for the biological growth. The constituent minerals of the parent rock (e.g., quartz in sandstone/quartzite) would exhibit their characteristic crystal forms if viewed microscopically, but macroscopic crystal forms are absent in a pebble.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for the pebble as a whole. The constituent minerals of the parent rock (e.g., quartz) lack distinct cleavage, exhibiting conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial (riverbeds, riverbanks), glacial (moraines, outwash plains), coastal (beaches, intertidal zones), desert (alluvial fans, desert pavements), or any terrestrial environment where rocks are exposed to weathering and biological colonization. Often found in areas with moderate moisture and sunlight.

Key Facts

Hardness

Variable, depending on the parent rock. For sandstone, typically 6-7 (Mohs) for quartz grains, but the cement can be softer. For quartzite, typically 7 (Mohs). The biological growth itself is soft and easily scratched.

Specific Gravity

Variable, depending on the parent rock. For sandstone, typically 2.2-2.8 g/cm³. For quartzite, typically 2.6-2.7 g/cm³. The biological growth adds negligible mass.

Crystal System

Not applicable for the pebble as a whole. Quartz, a common constituent, is trigonal.

Color

Highly variable, a combination of the parent rock's color and the biological growth's color.

Luster

Dull to earthy.

Transparency

Opaque for the pebble as a whole and the biological growth. Individual quartz grains can be translucent to transparent.

Fracture

Conchoidal to irregular for quartz-rich rocks. The biological growth does not fracture.

Cleavage

Absent in quartz. Other minerals in the parent rock may exhibit cleavage.

Composition

Primarily silicate minerals (e.g., quartz, feldspar) for the parent rock, with a superficial layer of organic compounds, pigments, and cellular structures from lichens, algae, or mosses.

Physical characteristics

• Crystal Habit: Not applicable for the pebble. Quartz grains are anhedral to subhedral.
• Cleavage Type: None for quartz. Other minerals may have cleavage.
• Fracture Type: Conchoidal to irregular for quartz-rich rocks.
• Tenacity: Brittle for the parent rock. The biological growth is flexible or friable.
• Luster Type: Dull to earthy.

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