Plasticity in diamond lattices has been studied extensively even if the phenomenon is considered difficult to achieve in actual diamond. Slip and cleavage occur on the {111}-planes and at low temperatures and pressures the latter prevails. Twinning and phase transformation are other mechanisms by which a material may yield to external stresses. Mechanical twinning (on {111}) has been reported for diamond lattices. The only phase transformation known to occur under normal circumstances is from the metastable diamond to the stable graphite. The author explores the possibility that also the cubic diamond to hexagonal lonsdaleite occurs, analogous to the martensitic FCC-HCP transformation in metals. A simple analysis shows that permanent deformations are only probable at elevated hydrostatic pressures in presence of a deviatoric stress. These conditions are attained during high-speed metal cutting with polycrystalline diamond (PCD) tools. SEM analysis of worn tools will demonstrate that certain accelerated wear processes in PCD can be attributed to the plastic deformation of the crystals at the tip. This implicates that the plasticity of diamond is a fairly common phenomenon with industrial relevance. Evidence for stress-induced phase transformation is presented also, but the exact nature of the phases that are formed remains in doubt.