osmium

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A bluish-white, hard metallic element, found in small amounts in osmiridium, nickel, and platinum ores. It is used as a platinum hardener and in making pen points, phonograph needles, and instrument pivots. Atomic number 76; atomic weight 190.2; melting point 3,000°C; boiling point 5,000°C; specific gravity 22.57; valence 2, 3, 4, 8. See Table at element.

Century Dictionary and Cyclopedia

1. n. Chemical symbol, Os; atomic weight, 191. One of the metals of the platina group. It does not occur native, but has been found to constitute a part of the native platina of all the platiniferous regions (South America, California, Australia, Russia), in the form of iridosmium, an alloy of the metals osmium and iridium. The specific gravity of the artificially obtained metal has been found to be 22.477; hence it is the heaviest of those bodies. It has never been fused. Its crystalline form is either that of the cube or that of a very obtuse rhombohedron. The crystals are of a bluish-white color, with a violet luster, and are harder than glass. Osmium is not used in the arts, except in the form of iridosmium, of which material the tips of gold pens are made.
2. n. This, the most refractory of the metals, has, though with difficulty, been completely fused into globules by means of the electric furnace.

Wiktionary

1. n. a heavy metallic chemical element (symbol Os) with an atomic number of 76

GNU Webster's 1913

1. n. (Chem.) A rare metallic element of the platinum group with atomic number 76. It is 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 190.2. Specific gravity 22.477.

WordNet 3.0

1. n. a hard brittle blue-grey or blue-black metallic element that is one of the platinum metals; the heaviest metal known