Quartzite

Hardness of 7 on the Mohs scale, typically white, gray, pink, or yellowish in color, vitreous to greasy luster, non-foliated massive crystal structure, concoidal fracture, specific gravity around 2.65.

Formed through the metamorphism of quartz-rich sandstone or chert under high temperatures and pressures, typically in tectonic mountain-building zones over millions of years.

Used in construction, as road aggregate, railroad ballast, dimension stone, and for manufacturing silica bricks and ferrosilicon.

Quartzite is extremely durable and resistant to chemical weathering, which is why quartz-rich ridges often form prominent mountain crests and hills.

Can be identified in the field by its inability to be scratched by a knife, sugary texture on freshly broken surfaces, and the fact that quartz grains break through the individual grains rather than around them (unlike sandstone). Found worldwide.