Sedimentary Rock

Pebble (river rock/beach pebble)

Pebble (clast size class)

Identified July 27, 2025More sedimentary rock →

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Physical properties

Hardness varies greatly depending on composition (generally 6-7 on Mohs scale for most common components). Color is typically grey, black, brown, or white, but can be variable. Luster is dull to earthy. Crystal structure is not applicable as it is a clastic sedimentary rock composed of mineral and rock fragments. No defined cleavage. Specific gravity varies with composition. These particular examples are water-worn and feel smooth. They are small, rounded, clastic sediment particles, which specifically classify them as pebbles.

Formation & geological history

Pebbles are clastic sedimentary particles, meaning they are fragments of pre-existing rocks and minerals. They form through the weathering and erosion of these larger parent rocks, followed by transport by agents such as water (rivers, streams, ocean waves) or ice (glaciers). During transport, the angular fragments are abraded and rounded, accumulating in riverbeds, lakeshores, beaches, and alluvial fans. Their geological age depends on the age of the source rocks and the time of deposition.

Uses & applications

Pebbles are widely used in landscaping (garden paths, decorative elements, drainage), construction (aggregate in concrete and asphalt, foundation material), and as decorative elements in aquariums and crafts. Smooth, attractive pebbles are also collected by hobbyists.

Geological facts

Pebbles are crucial indicators of past water flow and energy in geological environments. They can be found in a wide variety of depositional settings, from high-energy rivers and streams to beach environments. The degree of rounding indicates the amount of transport and abrasion the clast has undergone.

Field identification & locations

Pebbles are easily identified in the field by their rounded to sub-rounded shape and size (typically 4 mm to 64 mm in diameter, larger than granules and smaller than cobbles). They are commonly found in riverbeds, stream courses, beaches, and glacial deposits worldwide. To identify them, look for their characteristic smooth surfaces and rounded edges, which differentiate them from angular rock fragments. Their composition can vary, ranging from quartz and feldspar to fragments of basalt, granite, limestone, or chert, depending on the local geology.

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