Zirconium is a moderately soft, malleable metal that is highly resistant to corrosion. The metal itself has few uses, but its compounds are very important. Zircon, of ZrSiO4, is used as a refractory material and in ceramics. Zirconia, or ZrO2, is a very hard, very stable material with a very low thermal conductivity. It is used in hard, high quality ceramics including knives, as a specialized insulation material, and can be made into cubic zirconia, a low-cost diamond substitute.