The steel balls and beads - ball mill media - are preferred for contamination free and speedy (dry / wet) milling, grinding of solids, liquids dispersion and fine particle size reduction, homogenization of fillers and raw materials, attrition and pulverizing applications in ball, vibro, dyno, pearl, bead, pebble, attritor and small-media mills, vertical or horizontal. In the case of chocolates, the optimal size for cocoa particles is approximately 12 microns. Your tongue has the ability to feel texture. If the particles are too big, say 20 to 75 microns, then your tongue would sense an unpleasant roughness. If the particles are too small, say 1 to 9 microns, the cocoa particles would bind up more of the expensive cocoa butter. This would add cost to the process for more butter would have to be added to keep the product fluid.
The bead mills used for cocoa mass are vertical bead mills, using 5 to 6 mm or 1/4" and 3/16" diameter steel balls, beads or shots. Other sizes used vary from 8 mm to 11 mm or 5/16" to 7/16". Chocolate makers use stainless steel (inox) mediawhere absolutely no iron contamination is permissible and otherwise hardened chrome alloy steel balls.
Chocolate is very viscous. Because of the high density, no contamination, reasonable cost and lack of wear of steel media, the chocolate companies essentially use passivated steel balls as grinding media for chocolate paste in ball mills