metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, or white with reddish/brown stains from iron, Luster: vitreous to dull, Crystal structure: Trigonal (microcrystalline), Cleavage: none (conchoidal fracture), Specific gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, or white with reddish/brown stains from iron, Luster: vitreous to dull, Crystal structure: Trigonal (microcrystalline), Cleavage: none (conchoidal fracture), Specific gravity: 2.6-2.7
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause the sand grains to recrystallize and fuse together into a dense, hard rock. This specimen appears to be a river-worn or glacial erratic cobble.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and as a decorative stone in landscaping. High-purity quartzite is used to produce glass and silicon metal.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rocks during erosion, often forming the caps of hills and ridges. Unlike sandstone, quartzite fractures across the original sand grains rather than around them.
Field identification & locations
Identify by its extreme hardness (it will easily scratch glass and stainless steel) and its sugary, crystalline texture. Commonly found in mountain belts and glacial till. Collectors value smooth, translucent, or uniquely colored river-worn pieces.
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