Quartzite

Hardness: 7 (Mohs scale), Color: Smoky grey/white, Luster: Vitreous to dull, Crystal structure: Hexagonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7

Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure, typically during mountain-building events. The original quartz sand grains recrystallize and fuse into a solid mass.

Used extensively in construction as crushed stone for road gravel and rail ballast, as decorative building stone (countertops/floors), and in industrial applications for glass manufacturing.

Quartzite is so tough that it often outlasts the surrounding rocks; it commonly forms ridges and hilltops because it is highly resistant to both chemical and mechanical weathering.

Identify by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. In the field, look for a sugary or granular appearance where grains have fused together. Commonly found in folded mountain ranges.