Quartzite

Physical Properties

Hardness: 7 on Mohs scale; Color: typically white to gray, with yellowish-brown staining from iron oxides; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None, exhibits conchoidal or irregular fracture; Specific gravity: 2.6-2.7.

Formation & Geological History

Formed through the regional metamorphism of quartz-rich sandstone. Under intense heat and pressure, the individual quartz grains recrystallize and fuse together into a dense, interlocking matrix. This process can occur across various geological ages from the Proterozoic to more recent orogenic events.

Uses & Applications

Used widely as crushed stone for road construction, railroad ballast, and high-quality building stone/countertops. High-purity quartzite is used to produce silica sand for glassmaking and silicon metal production.

Geological Facts

Quartzite is so tough that it usually breaks through the quartz grains rather than along the boundaries between them, which is a key distinction from its parent rock, sandstone. It is often more resistant to weathering than the surrounding rock, often forming ridges and hilltops.

Field Identification & Locations

Identify in the field by its extreme hardness (it will easily scratch glass) and its grainy, sugary texture that resembles sandpaper but feels smooth and solid. It is found globally in mountainous regions and shield areas like the Appalachians or the Canadian Shield.

Identified on: 4/20/2026

Mode: Standard