sedimentary or igneous

River Stone (likely Siltstone or fine-grained Basalt)

Clastic Sedimentary (Siltstone) or Mafic Igneous (Basalt)

Hardness: 5.5-7 (Mohs scale); Color: charcoal grey to slate blue; Luster: dull/earthy; Crystal structure: microcrystalline or fine-grained clastic; Cleavage: none; Specific gravity: approx. 2.6-3.0

Hardness: 5
Color: charcoal grey to slate blue
Luster: dull/earthy

Identified March 15, 2026More sedimentary or igneous →

Explore River Stone (likely Siltstone or fine-grained Basalt) in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 5.5-7 (Mohs scale); Color: charcoal grey to slate blue; Luster: dull/earthy; Crystal structure: microcrystalline or fine-grained clastic; Cleavage: none; Specific gravity: approx. 2.6-3.0

Formation & geological history

Formed through the lithification of silt-sized particles or the cooling of lava. This specific specimen has been subject to mechanical weathering and fluvial erosion, rounding its edges over thousands of years in a high-energy water environment like a river or beach.

Uses & applications

Used in landscaping, decorative masonry, meditation stones (worry stones), and as aggregate for construction. Smooth specimens are often used in spa treatments (hot stone massage).

Geological facts

The 'smooth' texture is a product of 'abrasion,' where the stone collides with other rocks in moving water. While common, these stones are fundamental to the rock cycle, representing the transition from solid bedrock to sediment.

Field identification & locations

Identify in the field by its smooth, ellipsoidal shape and lack of visible large crystals. Common in riverbeds, glacial tills, and coastal areas. Collectors look for unique patterns or 'lucky' perfectly circular shapes.

Other sedimentary or igneous specimens

See all

Pyrite

Iron Sulfide (FeS2)

mineral

Pebble (likely a type of sandstone or quartzite pebble)

Clastic Sediment (specifically, a rudite-sized clast). If the pebble is composed of a specific rock type, its scientific name would be that rock type (e.g., 'Quartzite pebble', 'Granite pebble', 'Chert pebble'). Given its appearance, it is likely a metamorphic quartzite pebble or a hard sandstone pebble.

Sedimentary rock

Quartz Vein in Host Rock

Silicon Dioxide (SiO2) within Sedimentary or Igneous Matrix

igneous

Garnet

Garnet (X3Y2(SiO4)3)

mineral

Gneissic Schist

Gneiss-schist (Metamorphic rock)

metamorphic

Garnet in Host Rock

Garnet Group (A3B2(SiO4)3)

metamorphic