Iridium is a platinum group metal with atomic number 77.  Iridium is a very hard and brittle metal with silvery color.  The remarkable characteristics of Iridium is high melting point up to more than 2,400 degrees C and high corrosion resistivity not to be dissolved even by nitro hydrochloric acid under normal temperature, and those are the reasons why Iridium is necessary for growing crystal of high melting point.