metamorphic
Quartzite Cobble
Quartzite (SiO2)
Hardness: 7 (Mohs scale), Color: White to grey with brownish matrix, Luster: Vitreous to dull, Crystal Structure: Hexagonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: White to grey with brownish matrix, Luster: Vitreous to dull, Crystal Structure: Hexagonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.65
Formation & geological history
Formed through the metamorphism of quartz-rich sandstone under intense heat and pressure, typically in orogenic belts. This specific specimen has been significantly rounded through fluvial or coastal water erosion.
Uses & applications
Used in construction as road ballast, railway ballast, and decorative stone in landscaping. High-purity quartzite is used to produce ferrosilicon and silicon metal.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the caps of ridges and hills. When sandstone metamorphoses into quartzite, individual quartz grains recrystallize into a locked mosaic of crystals.
Field identification & locations
Identified by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. Commonly found in riverbeds and ancient glacial deposits. Look for granular texture and glassy luster on broken surfaces.
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