Quartzite

Hardness of 7 on the Mohs scale. Color ranges from white and gray to pink, red, or yellow due to iron oxide impurities. Luster is vitreous to greasy on fractured surfaces. Crystal structure is a dense, interlocking mosaic of quartz grains (non-foliated). Conchoidal fracture, no cleavage. Specific gravity is approximately 2.65.

Formed when quartz-rich sandstone or chert is subjected to high temperatures and pressures usually associated with tectonic compression in orogenic belts. This metamorphic process, known as quartz recrystallization, welds the quartz grains tightly together. Geological age ranges from Precambrian to Cenozoic.

Used widely in construction as crushed stone for road bases, railroad ballast, and dimension stone for building facades. Also used in the manufacture of silica brick, glass, and countertops, as well as by lapidaries for decorative carvings.

Unlike sandstone, which breaks around the individual grains, quartzite breaks straight through the quartz grains because the interlock is so strong. Some quartzites are so pure they contain over 99% silica.

Can be identified in the field by its extreme hardness (it will easily scratch glass and cannot be scratched by a steel knife) and the sugary texture of the broken surface where quartz grains fracture through rather than pull apart. Commonly found in mountainous regions and ancient cratons worldwide.