Quartzite

Typically Mohs hardness 7, white to cream with gray or rusty iron-oxide staining, and a sugary to glassy luster. It is massive and nonfoliated, with tightly interlocked quartz grains and no obvious cleavage.

Forms when quartz-rich sandstone is subjected to intense heat and pressure, commonly during mountain-building; individual specimens may span Precambrian to younger geological ages. This rounded piece was likely naturally weathered or water-worn.

Used as crushed stone, road aggregate, paving, building stone, and decorative material; attractive pieces are collected but usually have low gem value.

Quartzite is generally harder and more resistant to weathering than its sandstone precursor. Unlike marble, it does not fizz with dilute hydrochloric acid unless contaminated by carbonate minerals.

Identify it by testing whether grains fracture through rather than around the quartz, its very hard surface, and lack of layering or foliation; common sources include mountain belts and river or beach gravels worldwide. A definitive identification may require a hardness or thin-section test.