metamorphic
Quartzite
Meta-quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan; Luster: vitreous (glassy) to dull; Crystal structure: trigonal (recrystallized quartz grains); Cleavage: none, fractures conchoidally; Specific gravity: approx 2.65.
- Hardness
- 7 on Mohs scale
- Color
- typically white, gray, or tan
- Luster
- vitreous (glassy) to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan; Luster: vitreous (glassy) to dull; Crystal structure: trigonal (recrystallized quartz grains); Cleavage: none, fractures conchoidally; Specific gravity: approx 2.65.
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure, causing quartz grains to recrystallize and fuse together. Often associated with convergent plate boundaries and mountain-building events.
Uses & applications
Used as aggregate in road construction, railroad ballast, decorative countertop material (often sold as 'hard marble' or 'quartzite'), and as a source of silica in glass manufacturing.
Geological facts
Quartzite is so tough that it often outlasts the surrounding softer rocks, forming prominent ridges and peaks. It is frequently mistaken for marble, but unlike marble, it will not fizz in acid and is hard enough to scratch glass.
Field identification & locations
Identify in the field by its grainy, sugary texture and extreme hardness (it will easily scratch a knife blade). Commonly found in the Appalachian Mountains, Lake Superior region, and Brazil. Collectors look for high-purity or stained specimens.
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