metamorphic or igneous

Riverstone (Quartzite/Granite Cobble)

Silicon Dioxide (SiO2) based Quartzite or Silicate-rich Granite

Hardness: 6-7 on Mohs scale; Color: Gray, tan, or brown with dirty surface; Luster: Dull (weathered) to vitreous (on fresh break); Crystal Structure: Granular or microcrystalline; Cleavage: None; Specific Gravity: 2.6-2.7

Hardness: 6-7 on Mohs scale
Color: Gray, tan, or brown with dirty surface
Luster: Dull (weathered) to vitreous (on fresh break)

Identified March 22, 2026More metamorphic or igneous →

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Physical properties

Formation & geological history

Formed from the recrystallization of sandstone (quartzite) or cooling magma (granite). Its rounded, smooth elliptical shape is the result of long-term fluvial erosion in rivers or glacial transport during the Pleistocene to Holocene epochs.

Uses & applications

Used primarily in landscaping, concrete aggregate, road construction, and decorative garden features. High-quality smooth stones are sometimes used in hot stone massage or as painting surfaces for crafts.

Geological facts

Rocks like this are often called 'clasts' by geologists. Their roundness is a direct indicator of the distance they have traveled; the smoother and rounder the stone, the longer it has been tumbled by water or ice.

Field identification & locations

Identify in the field by its smooth, egg-like texture and high hardness (it will scratch glass). Commonly found in stream beds, alluvial fans, and glacial till. Collectors look for unique colors or perfectly symmetrical oval shapes.

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