From Halopedia, the Halo wiki
Osmium is a chemical element that has the chemical symbol Os in the Periodic Table of Elements. It has the atomic number of 76. Osmium is hard, brittle, with a blue-gray or blue-black transition metal in the platinum family, and is one of the densest natural elements. Competing for this status is iridium. Osmium is used in alloys with platinum, iridium and other platinum group metals. Osmium is found in nature as an alloy in platinum ore. Alloys of osmium are employed in fountain pen tips, electrical contacts, and in other applications where extreme durability and hardness are needed.