Quartzite

Hardness of 7 on the Mohs scale, color typically ranges from white to gray, pink, or reddish-brown due to iron oxide impurities. Luster is vitreous to greasy on fractured surfaces. Crystal structure is granular and interlocking quartz crystals. Cleavage is absent (it fractures conchoidally through crystals rather than around them). Specific gravity is around 2.65.

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

Used in construction, road aggregate, railway ballast, manufacturing of silica brick, and sometimes as a decorative stone or countertop material.

Because quartz is extremely resistant to weathering, quartzite ridges often stand out as prominent hills or mountain crests where surrounding softer rock has eroded away.

Can be identified in the field by its extreme hardness (scratches glass easily and cannot be scratched by a knife) and the fact that it fractures across the grains rather than breaking along sandstone-like bedding planes. Commonly found worldwide in ancient mountain belts.