Quartzite

Hardness: 7 (Mohs scale), Color: Brownish-red to tan, Luster: Dull to vitreous, Crystal structure: Granoblastic (non-foliated), Cleavage: None, Specific gravity: 2.6-2.7

Formed from the metamorphism of quartz sandstone under high heat and pressure, causing the original quartz grains to recrystallize and fuse into a solid mass. Frequently found in Precambrian and Paleozoic geological formations.

Used extensively in road construction (as crushed stone), as railroad ballast, for kitchen countertops, and as industrial sand for glass manufacturing.

Quartzite is so hard and durable that when it breaks, it fractures through the quartz grains rather than along the boundaries between them, unlike sandstone. It often forms the resistant caps of mountain ridges due to its weathering resistance.

Identify by its extreme hardness (it will easily scratch glass and stainless steel) and its grainy, sandpaper-like texture that doesn't actually rub off. This specimen shows significant rounding from water transport in a river or coastal environment.