The hardness of diamond is a difficult property to define for two reasons. First, hardness is a measure of plastic deformation but diamond does not plastically deform at room temperature. Second, hardness is measured using a diamond indenter. Measuring hardness in this case is, therefore, akin to measuring the hardness of soft butter with an indenter made of hard butter! Fortunately, the hardness of diamond is quite sensitive to orientation and using a Knoop indenter; a distorted pyramid with a long diagonal seven times the short diagonal, orientated in the hardest
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direction, gives somewhat repeatable results. The following hardness values have been obtained [Field 1983]:
(001) plane. [110] direction. 10,400 kg/mm2 (001) plane. [100] direction. 5,700 kg/mm2 (111) plane. [111] direction. 9,000 kg/mm2