a hard brittle blue-grey or blue-black metallic element that is one of the platinum metals; the heaviest metal known
source: WordNet 3.0
A rare metallic element of the platinum group, found native as an alloy in platinum ore, and in iridosmine. It is a hard, infusible, bluish or grayish white metal, and the heaviest substance known. Its tetroxide is used in histological experiments to stain tissues. Symbol Os. Atomic weight 191.1. Specific gravity 22.477.