metamorphic
Quartzite Cobble
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, white, and reddish-orange due to iron staining; Luster: Vitreous to dull; Structure: Granular, non-foliated; Cleavage: None (conchoidal fracture).
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, white, and reddish-orange due to iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, white, and reddish-orange due to iron staining; Luster: Vitreous to dull; Structure: Granular, non-foliated; Cleavage: None (conchoidal fracture).
Formation & geological history
Originally formed as quartz sandstone which underwent intense heat and pressure (metamorphism) in a tectonic environment, causing the quartz grains to recrystallize into a dense rock. The rounded shape indicates it was later eroded and transported by water (river or beach action).
Uses & applications
Used in construction as crushed stone for road ballast, as a decorative garden stone, and historically as a source of silica for glassmaking. Small water-worn cobbles are popular in landscaping.
Geological facts
Quartzite is so hard that it will typically break through the original quartz grains rather than around them. It is one of the most resistant rocks to chemical and physical weathering.
Field identification & locations
Identified in the field by its extreme hardness (cannot be scratched by a steel blade) and granular, sugary appearance when broken. Commonly found in old riverbeds and glacial deposits.
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