Ceramic aluminum oxide, a self-sharpening grit that is harder, tougher, and lasts longer than aluminum oxide * Type 29 reversed saucer-shape depressed center flap discs for grinding and finishing * Layered abrasive flaps wear away evenly * Fiberglass plate center is sturdy, so no backing pad is needed * Cooler grind prevents bluing, or the blue, brown, or yellow discoloration of the workpiece