The Collaborative International Dictionary of English v.0.48:

Iridosmine \Ir`i*dos"mine\, Iridosmium \Ir`i*dos"mi*um\, n.
   [Iridium + osmium.] (Min.)
   The native compound of iridium and osmium. It is found in
   flattened metallic grains of extreme hardness, and is often
   used for pointing gold pens.
   [1913 Webster]

WordNet (r) 3.0 (2006):

iridosmine
    n 1: a hard and corrosion resistant mineral that is a natural
         alloy of osmium and iridium (usually containing small
         amounts of rhodium and platinum); used in needles and pen
         nibs etc. [syn: osmiridium, iridosmine]