Shape of particles in DEM

Many granular materials encountered in engineering practice are of irregular shapes that are not essentially smooth or rounded. However, in DEM, grains are idealized as spheres and ellipsoids and their surface are assumed to be sufficiently smooth. As a result, I wonder why there is no model of  irregular shapes and what is the difficulty on implementing such a model? Is there any recent work aimed to simulate grains of irregular shape? Thanks a lot.