Quartzite

Hardness: 7 (Mohs scale), Color: light grey with yellowish/tan streaks, Luster: vitreous to dull, Crystal structure: granular, non-foliated, Cleavage: none (conchoidal fracture), Specific gravity: 2.6-2.7

Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause quartz grains to recrystallize and fuse together, typically in orogenic belts where tectonic plates collide.

Used in construction as crushed stone for roadbeds, as a decorative stone in landscaping, or as building stone. High-purity quartzite is also a source of silica for glass and silicon metal production.

Quartzite is so hard and chemically resistant that it often forms the ridges and peaks of mountain ranges because it weathers much more slowly than the surrounding rocks. It can be distinguished from sandstone because it fractures through the quartz grains rather than around them.

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and sugary, crystalline appearance on fresh surfaces. It is found globally in mountain ranges and ancient shield areas.