metamorphic
Quartzite
Metamorphosed Sandstone (SiO2)
Hardness: 7 on Mohs scale; Color: typically white to gray, but pink, red, or orange due to iron oxides; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None, breaks with a conchoidal fracture.
- Hardness
- 7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white to gray, but pink, red, or orange due to iron oxides; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None, breaks with a conchoidal fracture.
Formation & geological history
Quartzite forms when quartz-rich sandstone is subjected to high heat and pressure during regional metamorphism, causing the quartz grains to recrystallize and fuse into a dense, interlocking matrix.
Uses & applications
Used as crushed stone in road construction, as railroad ballast, for decorative building facades, countertops, and occasionally as a gemstone in cabochon form.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock in mountain ranges, forming prominent peaks and ridges. It is often confused with marble, but unlike marble, it will not react with acid.
Field identification & locations
Identify in the field by its extreme hardness (it will scratch glass) and the way it breaks through the quartz grains rather than around them. Common in folded mountain belts like the Appalachians and parts of the UK.
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