In general if a material is shearing along crystal plains the material is breaking apart. Also I am pretty sure that diamonds are polycrystalline meaning that t

In general if a material is shearing along crystal plains the material is breaking apart. Also I am pretty sure that diamonds are polycrystalline meaning that the a diamond is composed of many crystals with different orientations, so it is generally going to break on the boundaries between those crystals not the crystalline bonds themselves. For an example think of a polycrystal like steel (which I’m a little more familiar with), the atoms in the crystal lattice can be displaced a little (elastic deformation) or the crystalline grains can move or change shape (plastic deformation). Once the crystals (grains) themselves shear that means you’ve applied enough stress to break the crystal’s bonds and the material is going to break. Some materials are different though, and sometimes crystals can slip across grain boundaries. But the bonds between the crystals are generally weaker and more prone to move/change, generally polychrystals break along those boundaries first, before the crystals themselves break apart. This is probably very brittle (and hard) not because there are no shear plains, but because there are no mechanisms for the atoms to move around without breaking chemical bonds.