Quartzite

Hardness: 7 (Mohs scale), Color: Pinkish-grey to brown, Luster: Vitreous to dull, Crystal Structure: Hexagonal (microcrystalline), Cleavage: None (conchoidal fracture), 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 into a dense, hard rock. Often found in Precambrian or Paleozoic aged mountain belts.

Used as crushed stone for road construction, railway ballast, and as a decorative stone in landscaping and building facades. High-purity quartzite is used to produce glass and metallic silicon.

Quartzite is so tough that it often creates the rugged peaks of mountain ranges because it resists erosion much better than surrounding rocks. It is harder than a steel knife and will scratch glass easily.

Identify in the field by its sugary texture on fresh surfaces and its extreme hardness. It won't fizz with acid (distinguishing it from marble) and cannot be scratched by a steel nail. Common in mountainous regions of North America and Northern Europe.