The stuff you place to the tumbler exclusively to polish the rocks in the final phase is usually made of cerium oxide, tin oxide, chrome oxide, or aluminum oxide. It's a type of grit historically known as a polishing compound and is amazingly great. All of this can be done https://www.quora.com/profile/Trevor-Flatcher-2/Silicon-carbide-special-ceramics-in-modern-industry-in-a-variety-of-applications-Silicon-Carbide-Special-Ceramics-in-Mo