A limited range of materials is available for optical components of the highest quality. The quality which can be achieved depends not only on the material but also on the blank manufacturing method, the machining and finishing techniques and often, in the case of metals, on the properties of a more readily machinable or polishable overlayer. The materials and manufacturing methods employed will also determine whether the component will maintain its specification throughout its proposed lifetime during which it may be subjected to adverse mechanical and thermal stresses and corrosive conditions. For the highest quality optics, pure synthetic vitreous silica and the remelted quartzes are the preferred materials. If severe thermal changes are encountered in use, then ULE silica or the optical glass ceramics may be preferred despite their somewhat greater inhomogeneity. The best metallic optical components are made of beryllium coated with electroless nickel. This combination is inferior to glasses in its stability but has a superior stiffness to weight ratio and is superior to all but ULE silica and the glass ceramics in its response to rapid thermal fluctuations.