These 7/8" Medium, Blue Zirconia Alumina Sanding Discs are ideal for use with platinum, but will work well on gold and silver. These discs are resin-backed for superior durability and aggressively cut and grind metals with a uniform file-like finish. Last up to three times longer than conventional abrasives. Manufactured with a brass snap-on center which helps reduce vibration as they work. Use with a Snap-on Mandrel (sold separately item#: 840-3200). Made in USA.