metamorphic
Quartzite
Quartzite (SiO2-rich metamorphic rock)
Hardness: 7 on Mohs scale; Color: Gray, tan, or white; Luster: Vitreous to dull; Crystal structure: Non-foliated, granular; Cleavage: None/conchoidal fracture; Specific gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or white
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or white; Luster: Vitreous to dull; Crystal structure: Non-foliated, granular; Cleavage: None/conchoidal fracture; Specific gravity: 2.6-2.7.
Formation & geological history
Formed from the metamorphism of quartz sandstone under intense heat and pressure, typically during mountain-building events. Recrystallization locks quartz grains together, creating a very durable rock.
Uses & applications
Used as decorative stone, crushed stone for road construction, railway ballast, and occasionally in glassmaking if pure. Historically used for making tools because of its hardness and fracture pattern.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the peaks of hills and mountains because it resists weathering better than surrounding rocks. It can be distinguished from sandstone because it breaks across the sand grains rather than around them.
Field identification & locations
Identify by its extreme hardness (scratches glass) and granular texture. Look for it in glaciated areas or riverbeds as rounded cobbles. Found globally in ancient mountain belts.
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