Quartzite

Hardness of 7 on the Mohs scale, color typically white to gray but can include pink or red due to iron oxides, vitreous to sugary luster, crystal structure is trigonal/microcrystalline, concoidal fracture, specific gravity around 2.65.

Formed through the metamorphism of quartz-rich sandstone or chert under intense heat and tectonic pressure, typically in mountain-building environments over millions of years.

Used in construction, railway ballast, manufacturing of silica bricks, decorative aggregates, countertops, and occasionally in lapidary arts.

Because quartz is extremely resistant to weathering, quartzite ridges often form prominent, rugged mountain crests and resistant hills as the surrounding softer rocks erode away.

Can be identified in the field by its hardness (it will easily scratch glass and cannot be scratched by a steel knife) and the fact that it breaks across the quartz grains rather than around them like sandstone. Common worldwide, especially in areas with ancient mountain belts.