Iridium (Ir) is a dense, lustrous, and corrosion-resistant transition metal. It is one of the rarest elements in the Earth's crust and has a high melting point, making it ideal for high-temperature applications. Although not transparent, its optical properties are relevant in thin film coatings. Thin layers of iridium can be used as a coating material for mirrors in certain specialized optical systems, including X-ray telescopes. Moreover, the metal is often used in combination with other materials to form highly stable, temperature-resistant, and chemically inert optical surfaces. Its high cost often restricts its usage to specialized applications where other materials would fail. Given its excellent resistance to chemical corrosion, iridium is also used in electrodes for the chlor-alkali process and in certain types of spark plugs.