Quartzite is a very hard, non- foliated, metamorphic rock formed usually from quartz-rich sandstone. Under low to moderate grade metamorphism the quartz grains in sandstone fuse together. These bonds are so strong that when quartzite is broken it will break only along the original quartz grains.

Quartzite is usually white, but iron oxide may produce reddish, pinkish, purplr colors. Original sedimentary features such as cross-bedding may be preserved in some quartzites.

Information provided by Patrick M. Colgan, Northeastern University
http://www.casdn.neu.edu