metamorphic
Quartzite Cobble
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: yellowish-tan with reddish iron-oxide staining; Luster: vitreous to waxy; Texture: granular, microcrystalline; Cleavage: none (conchoidal fracture).
- Hardness
- 7 on Mohs scale
- Color
- yellowish-tan with reddish iron-oxide staining
- Luster
- vitreous to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: yellowish-tan with reddish iron-oxide staining; Luster: vitreous to waxy; Texture: granular, microcrystalline; Cleavage: none (conchoidal fracture).
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure, often during mountain-building events. This specific specimen has been further shaped and smoothed by water erosion in a river or coastal environment.
Uses & applications
Commonly used as crushed stone for road construction, railway ballast, and in the production of glass or silica-based ceramics. Smooth river-worn specimens are popular for landscaping and garden decor.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the caprock on mountain ridges. It is harder than steel, meaning a steel knife blade will not scratch it but will instead leave a metallic streak on the rock surface.
Field identification & locations
Identify in the field by its superior hardness (scratches glass) and its granular, sugary appearance when broken. It is extremely common in riverbeds and glacial deposits globally.
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