Iridium

Iridium is a very hard, dense, brittle, silvery-white metal belonging to the platinum group of metals. It is a chemical element with the symbol Ir and atomic number 77 in the periodic table. Iridium is found uncombined in nature with platinum and other platinum group metals in alluvial deposits.

Iridium is an extremely heavy metal. Its density is almost the same as Osmium - the most dense element known. Iridium is also the most corrosion-resistant metal known and cannot be attacked by any acid. 

Iridium is mainly used as a hardening agent for platinum alloys. The metal also used to make spark plugs and crucibles that require high temperatures.