It is also used for electrical contacts. Its principal use is as a hardening agent for platinum. With osmium, it forms an alloy which is used for tipping pens, and compass bearings. The specific gravity of iridium is only very slightly lower than that of osmium, which has been generally credited as being the heaviest known element. These values may be more reliable than actual physical measurements.
Natural iridium contains two isotopes.
Forty two other isotopes, all radioactive, are now recognized. At present, therefore, we know that either iridium or osmium, is the densest known element, but the data do not yet allow selection between the two.