metamorphic
Quartzite (River Cobble)
Metamorphosed Quartz Sandstone (SiO2)
AI-generated identification · may be incorrectHardness: 7 (Mohs scale), Color: tan to creamy white with internal crystalline translucency, Luster: Vitreous to greasy on fresh surfaces, Crystal structure: Trigonal/Hexagonal system (microcrystalline), Specific gravity: 2.6-2.7.
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Physical properties
Hardness: 7 (Mohs scale), Color: tan to creamy white with internal crystalline translucency, Luster: Vitreous to greasy on fresh surfaces, Crystal structure: Trigonal/Hexagonal system (microcrystalline), Specific gravity: 2.6-2.7.
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure, typically in orogenic (mountain-building) belts. This specific specimen has been further shaped and rounded by fluvial erosion in a river environment.
Uses & applications
Used in construction for road ballast, as a decorative garden stone, and industrially as a source of silica for glassmaking. Smooth cobbles are often used for lapidary tumbling or as worry stones.
Geological facts
Quartzite is so tough that it often outlasts the original mountains it formed in, eventually ending up as pebbles in rivers hundreds of miles away. It is often harder than the steel knives used to scratch it.
Field identification & locations
Identify in the field by its hardness (will easily scratch glass) and its lack of reaction to acid. Look for a sugary or crystalline texture on broken surfaces. Very common in glacial till and riverbeds worldwide.
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