For example, gypsum (Mohs' hardness number = 2) will scratch talc (Mohs' hardness number = 1). Talc, however, will not scratch gypsum. Glass is assigned a Mohs hardness number of 5.5 because it will scratch apatite (Mohs' hardness number = 5) but will not scratch orthoclase feldspar (Mohs' hardness number = 6).
