3 X 21 ZIRCONIA BELT 60 GRIT. Alumina Zirconia is two steps removed from the hardness of diamond. This premium material is extremely durable on metal, fiberglass, and hard woods. It is ideal for planing rough sawn lumber or roughing out shapes. Our belts have heavy cloth backing and taped butt joints. Belts can operate in either direction as they are bi-directional.