metamorphic
Quartzite Cobble
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: white to light gray with yellowish iron staining; Luster: dull to vitreous; Crystal structure: granoblastic (interlocking quartz grains); Cleavage: none; Specific gravity: approx. 2.65.
- Hardness
- 7 on Mohs scale
- Color
- white to light gray with yellowish iron staining
- Luster
- dull to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: white to light gray with yellowish iron staining; Luster: dull to vitreous; Crystal structure: granoblastic (interlocking quartz grains); Cleavage: none; Specific gravity: approx. 2.65.
Formation & geological history
Formed through the metamorphism of quartz-rich sandstone under intense heat and pressure, causing the original quartz grains to recrystallize and fuse into a dense, hard rock. This process often occurs during mountain-building events.
Uses & applications
Used extensively in construction as crushed stone for road beds, railway ballast, and as a decorative stone in landscaping. High-purity quartzite is used to produce silica sand for glassmaking and industrial abrasives.
Geological facts
Quartzite is so tough that it often survives intense weathering and transport in rivers better than most other rocks, resulting in the smooth, rounded 'cobble' shape seen here. It is harder than steel and will easily scratch glass.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its sugary, crystalline fracture surface. Commonly found in glaciated areas, riverbeds, and mountainous regions containing ancient metamorphic belts.
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