Quartzite

Hardness (7 on Mohs scale), color typically white to gray but can be brownish due to iron staining, vitreous (glassy) to dull luster, granular texture, lacks cleavage, specific gravity is roughly 2.65.

Formed through the regional metamorphism of quartz-rich sandstone. Under high heat and pressure, quartz grains recrystallize and fuse into a solid, interlocking network. Most formations range from Precambrian to Paleozoic ages.

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

Quartzite is so tough that it often outlasts the surrounding rocks during erosion, forming ridges and mountains. It is significantly harder and more durable than the sandstone from which it originated.

Identify in the field by its hardness (it will scratch glass) and its crystalline, sugary appearance. Unlike sandstone, quartzite breaks across the individual quartz grains rather than around them. Found globally in mountainous or metamorphic terrains.