Diamonds are extremely hard (resistant to pressure) but quite brittle (susceptible to shock). Sufficient impurity will compromise hardness, but I am finding it difficult to find out the effect on brittleness. Are synthetically-produced diamonds as hard as natural diamonds? is a related question.
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Diamonds tend to be naturally brittle and adding impurities generally doesn't help since they usually significantly increase the internal stresses in the diamond lattice. There has been one group that has reported growing "Ultra-tough" single crystal diamond synthesized from a methane/hydrogen/nitrogen gas mixture using chemical vapor deposition (CVD) ("Ultra-tough CVD diamond", see last paragraph). However, I don't believe that this claim has yet been studied and confirmed by any other CVD diamond group.
For your information, there are many forms of "diamond". There is, for example, single-crystal diamond, nano-diamond (polycrystalline diamond with nanometer-sized diamond grains), and "amorphous diamond" (which is amorphous carbon that is "diamond-like" in that it has a high fraction of sp3 bonds and also has somewhat "diamond-like" hardness, compressibility, and strength).
You didn't mention why you were interested in the brittleness/toughness of diamond, but if your interest or application isn't limited to pure diamond but also includes the possibility of diamond-based composite materials, I should mention that my own research involves fabricating tiny electrical and magnetic sensors in single-crystal diamonds for high pressure experiments ("Designer Diamond Anvils", LLNL). This line of work could lead to the development of diamond/metal composites which have diamond-like physical properties but much higher toughness or impact strength than pure diamond crystals owing to the presence of a network of tiny metal wires inside the diamond crystal which would act to distribute impact stresses and hinder the propagation of cracks in the composite.
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$\begingroup$ With a H2/CH4/N2 environment, I would expect either H entrainment in voids, N substituting for C in the lattice, or both. Either of these could possibly provide scattering of shear waves w/o weakening the lattice much. $\endgroup$ Commented Oct 15, 2015 at 6:48