Quartzite

Hardness: 7 (Mohs scale), Color: light grey to reddish-brown, Luster: dull to vitreous, Texture: granoblastic with visible crystalline grains, Cleavage: none, Specific Gravity: 2.6 - 2.8

Formed through the metamorphism of quartz sandstone. High heat and pressure in regional metamorphic environments cause the individual quartz grains to recrystallize and fuse together, creating a weld-like bond.

Used extensively in road construction, railroad ballast, building stone, and as a source of silica in glass manufacturing. Some varieties are used for landscaping or as decorative cladding.

Quartzite is so durable that it can survive the intense mechanical weathering of rivers and the chemical weathering of the atmosphere better than almost any other rock. Large mountains are often capped with quartzite because it resists erosion.

Identify in the field by its superior hardness (it will easily scratch glass) and its tendency to break across grain boundaries rather than around them. Often found in mountainous regions or as river-worn cobbles.