How to identify Polished Black Pebble

To correctly identify Polished Black Pebble (Sedimentary rock (e.g., Chert, Basalt, or other fine-grained dark 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

   Predominantly black, dark gray, or very dark brown. May show subtle hues of blue or green depending on the mineralogy.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Varies from dull to vitreous (glassy) or resinous, depending on the source rock and degree of polish. Obsidian will be distinctly glassy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth and rounded due to abrasion. The internal texture, if fractured, can be fine-grained (e.g., basalt, chert) or aphanitic (e.g., obsidian).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for a pebble; refers to the overall shape of the clast, which is typically sub-rounded to well-rounded.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for a pebble; the original rock may or may not exhibit cleavage (e.g., argillite may show slaty cleavage, but a pebble form obscures this).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in fluvial (riverbeds, floodplains), lacustrine (lake shores), glacial (moraines, outwash plains), and marine (beaches, continental shelves) environments where water or ice transport and abrasion are significant.

Key Facts

Hardness

Varies significantly (e.g., Chert: 6.5-7 Mohs; Basalt: 5-6 Mohs; Obsidian: 5-5.5 Mohs; Anthracite: 2.5-3 Mohs)

Specific Gravity

Varies significantly (e.g., Chert: 2.58-2.64; Basalt: 2.7-3.0; Obsidian: 2.35-2.60; Anthracite: 1.3-1.8)

Crystal System

Not applicable for a pebble; the constituent minerals of the source rock have their own crystal systems.

Color

Black, dark gray, very dark brown

Luster

Sub-vitreous, resinous, waxy, or dull, depending on the source rock and degree of polish.

Transparency

Opaque to translucent (e.g., thin edges of chert or obsidian may be translucent).

Fracture

Conchoidal (chert, obsidian), uneven (basalt), or splintery (some argillites).

Cleavage

Absent in the pebble form; original rock may have cleavage (e.g., argillite).

Composition

Highly variable. Examples: Chert (SiO2), Basalt (plagioclase, pyroxene, olivine), Obsidian (amorphous SiO2), Argillite (clay minerals, quartz, micas), Anthracite (carbon).

Physical characteristics

• Crystal Habit: Not applicable for a pebble; refers to the overall rounded shape of the clast.
• Cleavage Type: None observed in pebble form; depends on the source rock.
• Fracture Type: Conchoidal (chert, obsidian), uneven (basalt), splintery (argillite).
• Tenacity: Brittle (chert, obsidian, basalt), somewhat brittle (anthracite).
• Luster Type: Varies from dull to vitreous, resinous, or waxy.

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