Quartzite

Hardness: 7 (Mohs scale), Color: light grey to green-grey, Luster: Vitreous to dull, Structure: Granoblastic (crystalline), Cleavage: None, Fracture: Conchoidal/Irregular

Formed through the regional metamorphism of quartz sandstone. Intense heat and pressure cause the quartz grains to recrystallize and fuse together, often in orogenic (mountain-building) zones.

Used as crushed stone in construction, road ballast, railroad foundation, and occasionally as decorative stone or for making high-silica glass.

Quartzite is so tough that it often outlasts the surrounding rocks during erosion, creating prominent ridges or hills. It is harder than a steel knife and will easily scratch glass.

Identify by its extreme hardness and the fact that it breaks across sand grains rather than around them. Commonly found in riverbeds or near mountain ranges; this specimen appears to be water-worn (river-tumbled).