A noncrystalline, inorganic mixture of various metallic oxides fused by heating with glassifiers such as silica, or boric or phosphoric oxides. Common window or bottle glass is a mixture of soda, lime and sand, melted and cast, rolled or blown to shape. Most glasses are transparent in the visible spectrum and up to about 2.5 µm in the infrared, but some are opaque such as natural obsidian; these are, nevertheless, useful as mirror blanks. Traces of some elements such as cobalt, copper and gold are capable of producing a strong coloration in glass. Laser glass contains a small amount of didymium oxide. Opal glass is opaque and white, with the property of diffusing light. Some opal glass has a thin layer of opal material flashed onto the surface of ordinary glass. Tempered glass has a high degree of internal strain, caused by rapid cooling, which gives it increased mechanical strength.