metamorphic
Quartzite
Quartzite (composed primarily of SiO2)
Hardness: 7 on Mohs scale; Color: Grayish-green with brownish iron staining; Luster: Dull to vitreous; Crystal structure: Granoblastic (interlocking crystals); Cleavage: None; Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Grayish-green with brownish iron staining
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Grayish-green with brownish iron staining; Luster: Dull to vitreous; Crystal structure: Granoblastic (interlocking crystals); Cleavage: None; Specific gravity: 2.6-2.7
Formation & geological history
Formed through the metamorphism of quartz sandstone under high heat and pressure, typically during tectonic mountain-building events. The original sand grains recrystallize into a dense, interlocking mosaic of quartz.
Uses & applications
Commonly used as crushed stone for road construction, railway ballast, and as a decorative stone in landscaping. Its high silica content also makes it useful in the manufacturing of glass and ceramics.
Geological facts
Quartzite is so tough that it often survives intense weathering better than surrounding rocks, forming the caps of many mountain ridges. If you break it, it fractures across the original sand grains rather than around them.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its grainy but crystalline appearance. It is found globally in metamorphic belts. Collectors look for pieces with interesting mineral inclusions or vibrant iron coloring.
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