Attrition ball mills are an integral part of the sample crushing process when looking for diamonds. A typical preparation routine for diamond samples involves gradually crushing each sample down to < 2mms using several iterations of crushing to a smaller gap size then sorting  for any diamonds freed from the process then the material will be loading into autogenous ball mills where the samples are rolled for a minimum of 24 hours at moderate speed in order to loosen any diamonds trapped in the sample matrix. The goal with these mills is to provide gentle attrition of the samples so as not to break any diamonds that are near the surface of the surrounding matrix especially those diamonds that are resorbed. We generally do not add any ball charges to the mill when processing as the balls can cause diamond breakage at moderate speeds.
Over the 24-48 milling process, there is generation of the undersize particle size. This fraction is collected, washed, dry sieved and then processed through the HMC stage to recover the smaller diamonds and diamond indicator grains. This staged process is repeated until the oversize fraction weight is ~8-24 kgs and <2 mms in size. Although it is a time consuming step, it is the preferred method when processing kimberlitic / lamproitic rocks.