MOHS SCALE OF HARDNESS

 

The Viennese mineralogist Friedrich Mohs (1773-1839) invented the scratch hardness test. He defined scratch hardness as the resistance of a mineral when scratched with a pointed testing object. Mohs chose ten minerals of different hardness for comparison and graded these minerals one to ten. Each mineral in this series scratches the previous one. Minerals of equal hardness cannot scratch each other. By comparative application of Mohs' hardness scale the hardness (according to Mohs) of every gem can be determined. Stones with scratch hardness 1 and 2 are soft, 3 to 6 medium hard, over 6 hard. Minerals of Mohs' hardness 8 to 10 are also described as "hard gems." The luster and polish of gems of hardness below 7 can be damaged by dust as this may contain small particles of quartz (Mohs hardness 7). Such stones must be carefully handled when worn or stored so that they do not come into contact with any scratching objects. Relative and Absolute Hardness Scale Scratch hardness (Mohs) Mineral used for comparison Simple hardness tester Cutting hardness (Rosiwal) 1 Talc Can be scratched with fingernail 0.03 2 Gypsum Can be scratched with fingernail 1.25 3 Calcite Can be scratched with copper coin 4.5 4 Fluorite Easily scratched with knife 5.0 5 Apatite Can be scratched with knife 6.5 6 Orthoclase Can be scratched with steel file 37.0 7 Quartz Scratches window glass 120.0 8 Topaz Scratches window glass 175.0 9 Corundum Scratches window glass 1,000.0 10 Diamond Scratches window glass 140,000.0