metamorphic
Quartzite Cobble
Metamorphosed Quartz Sandstone
Hardness: 7 (Mohs scale), Color: light grey to tan with speckled inclusions, Luster: dull to vitreous, Structure: granular and non-foliated, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: light grey to tan with speckled inclusions, Luster: dull to vitreous, Structure: granular and non-foliated, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure causes recrystallization of the quartz grains, making it harder and more durable than the parent rock. This specimen has been rounded by fluvial (river) or glacial transport.
Uses & applications
Used as a decorative landscape stone, in construction as crushed stone for road beds, and historically for making stone tools due to its hardness and fracture patterns.
Geological facts
Quartzite is so strong that when it breaks, it fractures through the quartz grains rather than along the boundaries between them. It is one of the most resistant rocks to physical and chemical weathering.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. Look for it in riverbeds, glacial tills, and mountain ranges. It often has a 'sugary' texture on fresh surfaces.
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