metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale. Color: Off-white to light gray. Luster: Vitreous to waxy/dull when water-worn. Crystal Structure: Trigonal (microcrystalline interlocked quartz grains). Cleavage: None (conchoidal fracture). Specific Gravity: ~2.65.
- Hardness
- 7 on Mohs scale
- Color
- Off-white to light gray
- Luster
- Vitreous to waxy/dull when water-worn
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Physical properties
Hardness: 7 on Mohs scale. Color: Off-white to light gray. Luster: Vitreous to waxy/dull when water-worn. Crystal Structure: Trigonal (microcrystalline interlocked quartz grains). Cleavage: None (conchoidal fracture). Specific Gravity: ~2.65.
Formation & geological history
Formed when quartz-rich sandstone undergoes heat, pressure, and chemical activity during regional metamorphism, fusing individual quartz grains tightly together into a durable, dense crystalline matrix.
Uses & applications
Used extensively in construction as crushed stone for railway ballast, road base, and concrete aggregate. High-purity quartzite is used in glassmaking and silica refactories. Smooth pebbles are popular in decorative landscaping and lapidary tumbling.
Geological facts
Quartzite is extremely resistant to both chemical and physical weathering. Because it is so tough, quartzite outcrops often form prominent ridges and mountain peaks while softer surrounding rocks erode away over geological time.
Field identification & locations
Identified by its hard, sugary to interlocking texture that will easily scratch steel and glass. Unlike marble, which it can resemble, quartzite does not effervesce (fizz) in dilute acid. Commonly found in riverbeds, gravel pits, and metamorphic mountain belts worldwide.
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