Quartzite

Hardness of 7 on the Mohs scale. Colors range from white and gray to pink, red, and yellowish-brown depending on iron oxide impurities. Luster is typically vitreous to greasy on fractured surfaces. Crystal structure is non-foliated, composed of an interlocking mosaic of quartz grains. Conchoidal fracture, no distinct cleavage, and specific gravity of 2.65 to 2.75.

Formed when quartz-rich sandstone or chert is subjected to high temperatures and pressures usually associated with tectonic compression in orogenic belts. The original quartz grains recrystallize along with the cementing material, fusing them into a dense, extremely durable rock.

Used extensively in construction as crushed stone for road bases, railway ballast, and dimension stone for building facades, roofing, and flooring. Also used in the production of ferrosilicon, silica bricks, and manufacturing glass.

Unlike sandstone, which breaks around the individual grains, quartzite breaks straight through the quartz grains because the bonds created by metamorphism are stronger than the grains themselves. Famous occurrences include parts of the Appalachian Mountains and the Baraboo Range in Wisconsin.

Can be identified in the field by its extreme hardness (it will easily scratch glass and cannot be scratched by a knife) and the fact that it does not fizz with dilute hydrochloric acid. It often retains a sugary, glittering texture on freshly broken surfaces due to the interlocking quartz crystals.