metamorphic
River Cobble (Quartzite / Granitic Gneiss)
Metamorphic Quartzite / Granitic Gneiss
Hardness 6-7 on Mohs scale; dull to vitreous luster with glints of quartz/mica flakes; yellowish-brown, tan, gray, and whitish color; non-porous, rounded to sub-rounded texture due to river erosion; specific gravity ~2.6-2.7.
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Physical properties
Hardness 6-7 on Mohs scale; dull to vitreous luster with glints of quartz/mica flakes; yellowish-brown, tan, gray, and whitish color; non-porous, rounded to sub-rounded texture due to river erosion; specific gravity ~2.6-2.7.
Formation & geological history
Formed through deep regional metamorphism of sandstone or granitic rock subjected to intense heat and pressure, then weathered, detached, and smoothed by continuous aquatic transport in a stream or river bed over thousands of years.
Uses & applications
Used in landscaping, masonry, river rock features, decorative garden borders, crushed stone aggregate for construction, and as natural river stone decor.
Geological facts
River rocks acquire their smooth, rounded shapes solely through mechanical abrasion from sand, water, and collision with other rocks over long periods. Shiny flecks within the rock often consist of muscovite mica or quartz crystals.
Field identification & locations
Identified by its rounded cobble morphology, water-worn texture, high hardness (scratches steel), and visible mineral grains such as quartz, feldspar, and mica. Commonly found along active riverbeds, streams, and glacial outwash plains.
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