You see, that's always been something of a conundrum for me. Because magic exists as a tangible force in Equestria who's to say if what Sombra's using isn't some sort of solidified magical construct of even just regular earth given a matrix to fill in. But if only given your two choices, and taking your intended means (as diamonds are a form of crystal) I would hazard diamond due to the regal quality they have been given in general.