The stuff you put in to the tumbler especially to polish the rocks in the final phase is usually made of cerium oxide, tin oxide, chrome oxide, or aluminum oxide. It is a type of grit typically called a polishing compound and is extremely good. Further Fine Grits (400 �?600+): https://www.quora.com/profile/Trevor-Flatcher-2/Advantages-and-Challenges-of-Silicon-Carbide-Specialty-Ceramics-Macrocosm-Materials-Company-Overview-Founded-in-2016