metamorphic
Quartzite Pebble
Quartzite (SiO2)
Hardness: 7 (Mohs scale); Color: Off-white, yellowish, or tan due to iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None, exhibits conchoidal fracture under the surface; Specific gravity: 2.6 - 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Off-white, yellowish, or tan due to iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Off-white, yellowish, or tan due to iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None, exhibits conchoidal fracture under the surface; Specific gravity: 2.6 - 2.65.
Formation & geological history
Formed from the regional or contact metamorphism of quartz-rich sandstone. Under high heat and pressure, the individual quartz grains recrystallize and fuse together, creating a much harder rock. This specific specimen has been further shaped into a rounded pebble by fluvial (river) or glacial transport over thousands of years.
Uses & applications
Quartzite is used heavily in construction as crushed stone for road beds and railway ballast. Due to its durability and hardness, it is also used for decorative interior finishes, landscaping, and occasionally as a source of silica for glass manufacturing.
Geological facts
Quartzite is so hard that it will scratch glass and even steel. Unlike sandstone, which breaks around the individual sand grains, quartzite is so well-fused that it breaks through the grains themselves. Some quartzite formations contain up to 99% pure silica.
Field identification & locations
Identify in the field by its extreme hardness and granular, sugary appearance when broken. It is commonly found in ancient mountain belts and as river cobbles in areas with glacial deposits. Collectors should look for unique smooth shapes or translucent banding.
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