Quartzite

Hardness: 7 (Mohs scale), Color: White to light gray, Luster: Vitreous to dull, Crystal Structure: Trigonal (granular texture), Specific Gravity: 2.6-2.7

Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause the sand grains to recrystallize and fuse together. Typically Precambrian to Paleozoic in age.

Crushed stone for road construction, railway ballast, decorative aggregate, and as a source of silica for glass manufacturing.

Quartzite is so tough that it often survives intense weathering better than other rocks. During the Stone Age, it was occasionally used as a substitute for flint to create tools, though it is harder to flake.

Identify by its ability to scratch glass and its granular, sugary appearance. Unlike sandstone, quartzite will break through the grains rather than around them. Found globally in mountainous regions.