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]