Osmium. A sample of the transition metal Osmium (Os). Osmium is a hard, brittle, blue-grey or blue-black metal in the platinum family. It is the densest natural element, with a density of 22.59 g/cm3 (twice that of lead). Its alloys with platinum iridium, and other platinum group metals, are used in fountain pen tips, electrical contacts, and other applications where extreme durability and hardness are needed.