Multiple ultra-thin layers of different metals (such as gold or silver); oxides of such metals as titanium, chromium, aluminium, zirconium, or magnesium; or silica are vaporised by an electron beam in a vacuum chamber. The vapor then condenses on the surface of the glass in the form of a crystal structure. A protective layer of quartz crystal is sometimes added.[6] Other variants of such physical vapor deposition (PVD) coatings are also possible. The finished glass can have as many as 30[6] to 50 layers of these materials, yet the thickness of the total coating is approximately 30[6] to 35 millionths of an inch (about 760 to 890 nm). The coating that is created is very similar to a gemstone and, by careful control of thickness, different colors may be obtained.