Quartzite

Physical Properties

Hardness: 7 on Mohs scale; Color: white, gray, or tan; Luster: vitreous to dull; Crystal Structure: Trigonal (granular texture); Cleavage: none (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed through the regional or contact metamorphism of quartz-rich sandstone. Under high heat and pressure, the sand grains recrystallize and fuse together into a dense, crystalline rock.

Uses & Applications

Used heavily in construction as crushed stone for road base or railroad ballast, in architecture as dimensional stone, and occasionally in glassmaking if silica content is exceptionally high.

Geological Facts

Quartzite is so tough that it often survives intense erosion better than the rocks around it, leading to the formation of ridges and mountaintops. It is harder and more durable than the sandstone from which it originated.

Field Identification & Locations

Identify by its ability to scratch glass and its granular, sugary texture. Unlike marble, it will not fizz when exposed to weak acid. Common across the Appalachian Mountains and the Lake Superior region.

Identified on: 4/25/2026

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