metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, yellow, or rusty-orange due to iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 (Mohs scale)
- Color
- Tan, yellow, or rusty-orange due to iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, yellow, or rusty-orange due to iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Quartzite forms from the regional or contact metamorphism of quartz-rich sandstone. Under high heat and pressure, the sand grains recrystallize and fuse together. This specimen shows river-worn rounding, indicating it was transported by water long after its formation.
Uses & applications
Used in construction as a decorative stone, road ballast, or railway ballast. High-purity quartzite is used in the glass and silicate industries. Polished stones are sometimes used in landscaping or as aquarium decor.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock in weathering processes, appearing as prominent ridges or 'monadnocks' in the landscape. It is harder than steel and will easily scratch glass.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a knife) and its grainy, sugary texture on fresh breaks. Look for it in glaciated areas or riverbeds where hard rocks accumulate. This specimen's yellow/tan color comes from iron oxide infiltration into the crystal boundaries.
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