metamorphic
Quartzite Cobble
Quartzite (composed primarily of SiO2)
Hardness: 7 (Mohs scale); Color: Tan, brown, reddish-brown, and grey; Luster: Dull to vitreous; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brown, reddish-brown, and grey
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brown, reddish-brown, and grey; Luster: Dull to vitreous; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed from the metamorphism of quartz sandstone under high heat and pressure. This specific specimen is a water-worn cobble, likely eroded from ancient cratons or mountain belts and transported by river or ocean currents.
Uses & applications
Used as construction aggregate, railway ballast, landscaping stone, and architectural facing. In some cases, high-purity quartzite is a source of silica for glassmaking.
Geological facts
Quartzite is so tough that it often outlasts the mountain ranges it was formed in, becoming the primary component of river gravels. Ancient humans used quartzite to make stone tools because of its hardness and predictable fracture patterns.
Field identification & locations
Identify in the field by its ability to scratch glass and its grainy, sugary texture that resembles sandstone but doesn't crumble. Its rounded shape indicates significant travel in a high-energy aquatic environment like a beach or riverbed.
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