The theoretical strength of diamond has been calculated for the [100], [110
], and [111] directions using a first principles approach and is found to b
e strongly dependent on crystallographic direction. This elastic anisotropy
, found at large strains, and particularly the pronounced minimum in cohesi
on in the {111} direction, is believed to be the reason for the remarkable
dominance of the {111} cleavage plane when diamond is fractured. The extra
energy required to cleave a crystal on planes other than {111} is discussed
with reference to simple surface energy calculations and also the introduc
tion of bond-bending terms.