metamorphic
Quartzite Cobble
Meta-quartzite (SiO2)
Hardness: 7 (Mohs scale); Color: Brown, grey, tan, or white with a weathered exterior; Luster: Dull to vitreous; Crystal structure: Non-foliated, granular; Cleavage: None (conchoidal fracture); Specific gravity: 2.6 - 2.7.
- Hardness
- 7 (Mohs scale)
- Color
- Brown, grey, tan, or white with a weathered exterior
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Brown, grey, tan, or white with a weathered exterior; Luster: Dull to vitreous; Crystal structure: Non-foliated, granular; Cleavage: None (conchoidal fracture); Specific gravity: 2.6 - 2.7.
Formation & geological history
Formed through the regional metamorphism of quartz sandstone. Originally deposited as sand, heat and pressure caused quartz grains to recrystallize. This specific specimen’s rounded shape indicates it was later eroded and tumbled in a high-energy water environment, such as a riverbed or glacial stream.
Uses & applications
Used primarily in construction as crushed stone or road ballast. High-purity quartzite is used to produce silica sand for glassmaking and silicon for electronics. Rounded cobbles are often used in decorative landscaping and masonry.
Geological facts
Quartzite is so tough that it usually breaks through the quartz grains rather than along the boundaries between them, unlike sandstone. It is one of the most durable rocks on Earth's surface and is often found as 'erratics' transported hundreds of miles by glaciers.
Field identification & locations
Identify in the field by its inability to be scratched by a steel knife and its lack of reaction to acid. Look for a sugary or sparkly texture on a fresh break. Commonly found in mountainous regions, riverbeds, and glacial deposits globally.
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