metamorphic
Quartzite (River Pebble)
Metamorphosed Quartz Sandstone (SiO2)
Hardness: 7 (Mohs scale); Color: Grayish-white with yellow/orange iron staining; Luster: Vitreous to dull; Crystal structure: Granoblastic (crystalline); Cleavage: None (conchoidal fracture)
- Hardness
- 7 (Mohs scale)
- Color
- Grayish-white with yellow/orange iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Grayish-white with yellow/orange iron staining; Luster: Vitreous to dull; Crystal structure: Granoblastic (crystalline); Cleavage: None (conchoidal fracture)
Formation & geological history
Formed when quartz-rich sandstone is subjected to high heat and pressure, causing the sand grains to recrystallize into a dense, hard metamorphic rock. This specific specimen has been rounded by fluvial (river) erosion.
Uses & applications
Construction aggregate, railroad ballast, decorative landscaping, and occasionally used as a source of high-purity silica for glassmaking.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the caps of mountains and ridges. It is harder than steel; a steel knife will not scratch it, but the rock will scratch the knife.
Field identification & locations
Identified in the field by its sugary texture, extreme hardness, and lack of reaction to acid. Commonly found in riverbeds, glacial deposits, and ancient mountain belts.
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