Quartzite

Hardness: 7 (Mohs scale), Color: White, gray, or tan, Luster: Vitreous to dull, Crystal structure: Trigonal (granular texture), Cleavage: Indistinct/conchoidal fracture, Specific gravity: 2.6-2.7

Formed through the regional or contact metamorphism of quartz-rich sandstone. It is typically found in mountainous regions or stable continental interiors (cratons) where ancient sedimentary rocks were subjected to heat and pressure.

Used in construction as crushed stone for road ballast, as a decorative building stone, in high-end countertops, and as a source of silica for glass and silicon metal production.

Quartzite is so hard and chemically resistant that it often forms the tops of hills and ridges because it resists weather and erosion better than the rocks surrounding it.

Identify in the field by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. Unlike sandstone, quartzite breaks through the quartz grains rather than around them, resulting in a sugary or smooth fractured surface.