metamorphic
Quartzite Cobble
Quartzite (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, brown, or rusty due to iron staining; Luster: Dull to vitreous; Crystal structure: Non-foliated, granular; Specific Gravity: 2.6-2.7.
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brown, or rusty due to iron staining
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brown, or rusty due to iron staining; Luster: Dull to vitreous; Crystal structure: Non-foliated, granular; Specific Gravity: 2.6-2.7.
Formation & geological history
Formed when quartz-rich sandstone is subjected to intense heat and pressure (metamorphism), causing grains to recrystallize and fuse into a solid rock. This specimen shows signs of river or glacial transport through its rounded edges.
Uses & applications
Used in construction as road ballast, railway ballast, and decorative garden stone. Pure varieties are used in glass making and ceramics.
Geological facts
Quartzite is so hard and durable that it often survives intense mechanical weathering, remaining as cobbles in riverbeds long after softer rocks have eroded away. It will scratch glass easily.
Field identification & locations
Identify in the field by its extreme hardness and grainy, sugary texture under the surface. It is commonly found in glacial till and riverbeds across North America and Europe.
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