metamorphic
Quartzite (with Iron Staining)
Metamorphosed Quartz Sandstone (SiO2)
Hardness: 7 (Mohs scale), Color: White to tan with orange/brown iron staining, Luster: Vitreous to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None/Conchoidal fracture, SG: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: White to tan with orange/brown iron staining, Luster: Vitreous to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None/Conchoidal fracture, SG: 2.65
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone. High heat and pressure cause quartz grains to recrystallize and fuse together, creating a very hard, dense rock. The orange staining is likely from iron oxide (limonite or hematite) infiltration.
Uses & applications
Commonly used as crushed stone in construction, road ballast, and as a decorative landscaping stone. High-purity quartzite is used to produce silica sand for glassmaking.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock during erosion, forming ridges and mountain resistant caps. It is one of the hardest common rocks found on Earth's surface.
Field identification & locations
Identify in the field by its inability to be scratched by a steel knife and its grainy, sugary appearance on fresh surfaces. Unlike sandstone, quartzite breaks through the grains rather than around them.
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