Tungsten carbide grinding balls are twice the density of stainless steel.  Thus, for samples which are very resilient, 3/8" tungsten carbide grinding balls are a good alternative to stainless steel balls.  These balls should be used with 4 ml polycarbonate vials, or the larger 15 ml vials, in the standard vial set format.  Balls should be cleaned and re-used.  Sold in packages of 25 balls.