Quartzite

Mohs hardness of 7, vitreous to greasy luster, massive crystal structure with interlocking quartz grains, conchoidal fracture, specific gravity around 2.65.

Formed from quartz-rich sandstone or chert under high heat and pressure during regional or contact metamorphism.

Used in construction, railroad ballast, crushed stone for road aggregates, decorative tiles, and occasionally in sculpture.

Quartzite is extremely durable and resistant to chemical weathering, often forming prominent mountain ridges because it erodes much slower than surrounding rocks.

Can be identified in the field by its sugary texture on broken surfaces, extreme hardness (scratches glass easily), and lack of reaction to acid. Commonly found globally in mountainous regions and ancient orogenic belts.