A table of comparative hardness devised by the Austrian mineralogist, Friedrich Mohs. Based on the ten relatively common minerals which are ranked from 1 to 10 in order of increasing hardness: (1) talc (2) gypsum (3) calcite (4) fluorite (5) apatite (6) feldspar (7) quartz (8) topaz (9) corundum (10) diamond. A fingernail will scratch gypsum and softer substances. Ordinary window glass is slightly softer than feldspar. Almost all-important numbers in the scale indicate only the order of hardness and bear no relationship to quantitative hardness. That is, there is a greater difference between the hardness of corundum and diamond (9 and 10) than between 1 and 9.