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Tuesday, 18 September 2007

directional hardness. The relative hardness of diamond varies in dif­ferent directions. Because of dia­mond's covalent bonding which in­volves sharing of electrons, hard­ness is spatially directed. Thus hard­ness in diamond depends upon the number of bonds per unit area (bond density) and, therefore, varies with crystallographic direction. Conse­quently, the greater the number of chemical bonds in a given volume or direction, the harder is the mineral or directional property. The order of relative hardness in diamond from hardest to softest is: 1) cube 45° to axis; 2) dodecahedron 90° to axis; 3) octahedron towards cube; 4) oc­tahedron to dodecahedron; 5) cube parallel to crystal axis; 6) dodeca­hedron parallel to crystal axis. These

relative differences in hardness vary from about ten times to one hundred times; consequently the practical polishing directions are 5 and 6 above, since polishing against the lattice is harder than polishing with it. There is no soft direction on the oc­tahedral plane.

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