Hardness of minerals and other materials is usually inferred by testing if some standard minerals are able to scratch or indent others. An arbitrary (considered now as standard) scale was developed by a German mineralogist, Friedrich Moh in 1812. He arranged ten minerals in order of hardness. The standard minerals making up the hardness scale are: Talc (1), Gypsum or rocksalt (2), Calcite (3), Fluorite (4), Apatite (5), Feldspar or Orthoclase (6), Quartz (7), Topaz (8), Corundum (9) and Diamond (10). The serial numbers in parentheses indicate the hardness values of the corresponding minerals. Note that a mineral of larger hardness in this scale will scratch those lower in the scale. This scale is approximately linear up to corundum, but diamond is approximately 5 times harder than corundum. Tabor1 showed the relation log H = nM to hold for the first nine substances, where H is the indentation hardness in kg/mm2, M is Moh's number and n is about 1.6. The Moh's scale has resulted in a simple method to determine hardness of minerals, a procedure followed by geologists. According to this procedure, 'material of hardness 1 gets easily scratched by fingernail, that with hardness 2 gets scratched by fingernail, that with hardness 3 scratches a penny, that with hardness 4 scratches a penny if pressed hard, that with hardness 5 scratches steel easily, that with hardness 6 scratches glass if pressed hard and that with hardness 7 scratches glass easily. Materials with hardness 8, 9 and 10 are not common minerals'. It may be noted that hardness is measured, in addition to scratch procedure as outlined above, by two other methods (a) by the diameter of the indentation made by a hardened steel sphere (Brimmel) and (b) height of a rebound of a small drop hammer (Shore scleroscope). The latter is referred as dynamic hardness measurement also. In fact, hardness numbers are referred to by different names/methods of measurement: Brinnel, dynamic, Firth, Knoop, Ludwik, Monotron, pendulum, Rockwell, scratch, Shore, Vickers, etc. Reference 1 provides complete information on these and other aspects related with hardness. Certain empirical relations exist among mechanical physical properties and hardness, e.g. Brinnel hardness is nearly three to five times the ultimate tensile strength of metals and is given in units of kg/mm2 [1 kg/mm2 = 9.80665 ´ 106 Pascals (Pa) and 1 standard atmosphere = 101.325 kilopascals (kPa); 1 gigapascal (GPa) » 10 kilobars; sometimes hardness is given in terms of GPa units also] or in tons/in2. Vicker's hardness is related to yield stress and so on and one can interrelate or compare different hardness values obtained by different methods. Therefore one notes that according to this scale, diamond is the hardest among all minerals.