These are yttria-stabilised tetragonal zirconia beads: 95% ZrO2 with about 5% Y2O3 as the phase stabiliser, isostatically pressed and sintered to a fine grain size. Yttria keeps the tetragonal phase in place, which is what gives the bead its toughness, and toughness is why a zirconia bead resists fracture under continuous impact while a brittle media crumbles into the charge. The result is a bead with high density, high hardness and a low wear rate at the same time. That combination is rare: alumina media can match the hardness but not the density or the toughness, and steel media can match the density but not the chemical inertness or the white colour.