sandstone that has been converted into highly resistant solid quartz rock; may be snowy white, pink, or gray; smooth structure; varieties include orthoquartzites and metaquartzites; yields thin, very barren soil; projects out of the Earth as hills or mountain masses; parts of Appalachian Mountains composed of tilted beds of quartzite; impure quartzites include graywackle, litharenite, and sandstone; source of silica for metallurgical purposes; also quarried for paving blocks, road metal, railroad ballast, and roofing granules