Quartzite

Hardness: 7 (Mohs scale), Color: White, gray, or tan, Luster: Vitreous to dull, Crystal structure: Trigonal (granular texture), Cleavage: None, Specific gravity: 2.6-2.7

Formed through the regional metamorphism of quartz-rich sandstone under intense heat and pressure, typically during tectonic plate collisions and mountain-building events.

Used extensively in construction as crushed stone for roadbeds, railroad ballast, and decorative stone. High-purity quartzite is used to produce glass and silicon metal.

Quartzite is so durable that it often forms the crests of high mountain ridges because it resists weather and erosion better than the surrounding rock. It is often harder than the steel of a knife blade.

Can be identified by its sugar-like granular texture and its ability to scratch glass. In the field, look for it in metamorphic belts and transition zones between sandstone and granite.