metamorphic
Quartzite (River Cobble)
Recrystallized Quartz (SiO2)
Hardness: 7 (Mohs), Color: Tan to reddish-brown (likely stained by iron oxides), Luster: Dull to vitreous, Crystal structure: Granular, Specific gravity: ~2.65
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Physical properties
Hardness: 7 (Mohs), Color: Tan to reddish-brown (likely stained by iron oxides), Luster: Dull to vitreous, Crystal structure: Granular, Specific gravity: ~2.65
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure. This specimen shows significant rounding and smoothing from fluvial (river) erosion.
Uses & applications
Commonly used as construction aggregate, railway ballast, and in crushed form for road building. Dense, hard varieties are used as ornamental stone.
Geological facts
Quartzite is so hard that it will scratch glass and even metal knives. It is much harder and more durable than the parent sandstone from which it originated.
Field identification & locations
Identify by its hardness (cannot be scratched by a steel blade) and grainy 'sugary' texture on fresh breaks. Found globally in riverbeds and mountainous glacial deposits.
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