The fracture toughness, KIC, of polycrystalline silicon carbide is shown to be increased more than 100%, up to 7.17 MPa·m1/2, by the addition of dispersed nano-diamond particles. This KIC increase is critically dependent on the volume percent and the particle size of the diamond particle. The extremely small size of these isolated diamonds represents, by several orders of magnitude, the smallest particulate toughening agent added as a dispersed powder, as opposed to internal nucleation, ever reported. The toughening mechanisms which are considered are transformation toughening, microcrack toughening, and crack deflection. The significance of the non-cubic diamond polytypes in transformation toughening is discussed. Quantitative analysis of the hexagonal and cubic polytype distribution by Rietveld powder X-ray diffraction profile fitting of the explosively produced diamond of the type used for toughening enhancement is reported. Besides increasing the fracture toughness, the added diamond increases the thermal conductivity by 70% according to theory. It is also expected that the ΔT required to thermal shock the carbide will also be increased. Work is continuing to measure these and other physical properties of these unique composites