English: Hafnium and zirconium are two of the elements that are most similar to each other. Therefore they are hard to separate. The silvery, heavy hafnium so far is used only for a few special technical applications. Hafnium carbide (HfC) and tantalum hafnium carbide (Ta₄HfC₅) are very hard and mechanically enduring, the latter has with more than 4000°C the highest melting point of all materials. After being exposed to air for more than a year, the surface is slightly oxidized and therefore darker and less glossy.