Rock Identifier
River Rocks (Black) (Various lithologies (e.g., basalt, chert, obsidian for black varieties), mechanically rounded clasts) — Sedimentary
Sedimentary

River Rocks (Black)

Various lithologies (e.g., basalt, chert, obsidian for black varieties), mechanically rounded clasts

Hardness: Varies depending on source rock (e.g., basalt 6, chert 7, obsidian 5-5.5); Color: Predominantly black, can be dark grey; Luster: Variable, from dull to vitreous when wet or polished; Crystal Structure: Not applicable as they are rock fragments, inner structure dependent on source rock;…

Hardness
Varies depending on source rock (e
Color
Predominantly black, can be dark grey
Luster
Variable, from dull to vitreous when wet or polished
Identified More sedimentary →
Explore River Rocks (Black) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App StoreGoogle Play

Physical properties

Hardness: Varies depending on source rock (e.g., basalt 6, chert 7, obsidian 5-5.5); Color: Predominantly black, can be dark grey; Luster: Variable, from dull to vitreous when wet or polished; Crystal Structure: Not applicable as they are rock fragments, inner structure dependent on source rock; Cleavage: Not applicable; Specific Gravity: Varies (e.g., basalt ~2.9-3.2, chert ~2.6-2.8, obsidian ~2.3-2.6)

Formation & geological history

River rocks are formed by the erosion and transportation of larger rocks by flowing water, typically in rivers and streams. Over long periods, the abrasive action of water, sediment, and other rocks causes them to become rounded and smoothed. Their geological age varies widely as they are fragments of older rocks from the river's drainage basin.

Uses & applications

Commonly used in landscaping (garden borders, decorative features, mulching), aquariums, as decorative elements in crafts, concrete aggregates, and for various architectural applications.

Geological facts

The black color often comes from source rocks like basalt, obsidian, or highly metamorphosed shales. Their smooth, rounded shape is a typical indicator of significant water-borne erosion, making them distinct from angular, freshly broken rock fragments.

Field identification & locations

To identify in the field, look for smooth, rounded surfaces, often with a slightly polished appearance when wet. They will be found in and along riverbeds, stream banks, and floodplains. Common locations are worldwide wherever rivers and streams flow. Tips for collectors include observing their shape and size consistency, and noting the underlying rock types in the area to understand their composition.

More like this

Other sedimentary specimens

Sandstone or Siltstone (Arenite (if sandstone)) — Sedimentary

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary
Cone-in-cone structure (Limestone) (Cone-in-cone structure (Secondary Sedimentary Structure)) — sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary
Chert Breccia (Brecciated Chert (Microcrystalline Silica)) — sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary
Sandstone Grain (Clastic Sedimentary Rock Grain (SiO2 dominated)) — sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary
Shale or Slaty Mudstone (Argillaceous sedimentary rock) — sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary
Sandstone (Arenite (primarily SiO2)) — sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary