When stainless steel is impractical and zirconium oxide unnecessary, 4 mm acid washed silica beads are a good choice for standard laboratory applications.  Silica beads are more economical than zirconium silicates and oxides, while often as effective.  Acid washed silica beads are treated with acidified water to remove fine particles, dust, and other impurities typically found in untreated beads.  They are further baked to drive off organic impurities.