How to identify Pebble

To correctly identify Pebble (Sedimentary rock (likely shale or slate)), 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 source rock. Shale pebbles can be grey, black, brown, red, or green. Slate pebbles are typically dark grey, black, greenish-grey, or purplish-grey.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable. Shale pebbles often have a dull to earthy luster. Slate pebbles exhibit a dull to sub-vitreous luster, sometimes slightly satiny on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rounded to sub-rounded clastic texture. If the pebble itself is shale, it will feel smooth and fine-grained. If it's slate, it will feel smooth and very fine-grained, potentially with a slightly gritty feel due to microscopic quartz grains.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for a pebble as a whole, which is a detrital fragment. The constituent minerals within a shale or slate pebble would be microscopic and anhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   A pebble, as a clast, does not inherently have cleavage. However, if the pebble is composed of shale, it may exhibit fissility (the tendency to split into thin layers parallel to bedding). If the pebble is composed of slate, it will exhibit slaty cleavage, allowing it to split into thin, strong, parallel sheets, often at an angle to the original bedding.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Pebbles are found in high-energy sedimentary environments such as riverbeds, beaches, alluvial fans, and glacial tills. They are also found within lithified sedimentary rocks like conglomerates. Pebbles composed of shale or slate would indicate a source area where these rocks were exposed and undergoing erosion.

Key Facts

Hardness

Variable, depending on composition. Shale pebbles typically 2.5-4 on Mohs scale. Slate pebbles typically 3-5 on Mohs scale.

Specific Gravity

Variable. Shale pebbles typically 2.0-2.8 g/cm³. Slate pebbles typically 2.7-2.9 g/cm³.

Crystal System

Not applicable for a pebble as a whole. Constituent minerals are typically anhedral and microscopic.

Color

Highly variable, reflecting the source rock. Common colors include grey, black, brown, red, green (shale); dark grey, black, greenish-grey, purplish-grey (slate).

Luster

Dull to earthy (shale); Dull to sub-vitreous, sometimes satiny on cleavage surfaces (slate).

Transparency

Opaque.

Fracture

Variable. Shale pebbles may exhibit irregular to conchoidal fracture. Slate pebbles typically exhibit a splintery or platy fracture along cleavage planes, and irregular fracture perpendicular to cleavage.

Cleavage

A pebble itself does not have cleavage. If composed of shale, it may show fissility. If composed of slate, it will exhibit slaty cleavage.

Composition

Pebbles are rock fragments, so their composition is that of the parent rock. Shale pebbles are composed primarily of clay minerals (kaolinite, illite, smectite), quartz, and feldspar, with minor organic matter and other detrital grains. Slate pebbles are composed of fine-grained micas (muscovite, paragonite), chlorite, quartz, and sometimes pyrite or graphite.

Physical characteristics

• Crystal Habit: Not applicable for a pebble. Constituent minerals are microscopic and anhedral.
• Cleavage Type: Not applicable for a pebble as a whole. If composed of shale, fissility (parting along bedding planes). If composed of slate, slaty cleavage (excellent, planar parting due to preferred orientation of platy minerals).
• Fracture Type: Irregular to conchoidal (shale); Splintery/platy along cleavage, irregular otherwise (slate).
• Tenacity: Brittle.
• Luster Type: Dull to earthy (shale); Dull to sub-vitreous (slate).

Explore Pebble