The name “ball” or “pebble” refers to the type of grinding media that is introduced into the mill. The media when combined with ores or other subtances accelerate particle size reduction by virtue of their impacting on the particles as the mill rotates. The grinding action is as described in our article about how grinding mills work. Balls are essentially the same but normally come in metallic materials and are typically 1” (25mm) or larger depending on grind-ability factors and desired particle size reduction (PSD) of the product. Where very fine grinding is needed in cases such as paint pigments or high quality optical silica, pebbles are considered. Pebbles come in both metallic and non-metallic materials such as aluminum oxide. Non-metallic pebbles are typically used where the product cannot be contaminated with metal particles resulting from the inherent erosion of the grinding media.