Alumina, also called aluminium oxide, is a synthetically produced aluminium oxide. It is a white or nearly colourless crystalline substance that is used as an engineering ceramic due to its high performance at a cost-effective price. It is also an active agent in chemical processing. Alumina is made from bauxite, a naturally occurring ore containing variable amounts of hydrous (water-containing) aluminium oxides. Free Al2O3 occurs in nature as the mineral corundum and its gemstone forms, sapphire and ruby; these can be produced synthetically from alumina. Activated alumina is a porous, granular substance that is used as a substrate for catalysts and as an adsorbent for removing water from gases and liquids. Smelter-grade alumina accounts for 90 % of all alumina produced; it is transported to aluminium plants, where it is electrolyzed into aluminium metal. Calcined alumina is made into a variety of ceramic products such as insulators, bone and dental implants, laboratory ware, sandpaper grits and grinding wheels, and refractory linings for industrial furnaces. These products exhibit the properties for which alumina is well known, including low electric conductivity, resistance to chemical attack, high strength, and extreme hardness. Zirconia-toughened alumina is a modified, stronger grade of alumina. Alumina components can be manufactured by several different methods including pressing, extruding, slip casting and injection molding. Its combination of high thermal conductivity and low thermal expansion imparts good thermal shock resistance.