Osmium is one of the rarest and hardest metals in the world. It is a hard, brittle, bluish-white transition metal in the platinum group of the periodic table. It is the densest naturally occurring element, with a density of 22.
From Wikipedia
Osmium is a chemical element; it has symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group. Osmium has the highest density of any stable element and one of the lowest abundances in the Earth's crust. Manufacturers use alloys of osmium with platinum, iridium, and other platinum-group metals for fountain pen nib tipping, electrical contacts, and other applications that require extreme durability and hardness.