Pyrophoric: liable to ignite spontaneously when exposed to air.  You know, like..  Charlie Sheen. 

While a pyrophoric chemical might ignite, a pyrophoric alloy will produce sparks when struck.  Lighter flints, for example, are pyrophoric.  The flintlock mechanism in old firearms used pyrophoric metals.

From Greek πυροφόρος (purophóros), meaning, fire-bearing.  This word derives from two Greek words.  Πῦρ (pûr), fire and φέρω (phérō) meaning, I carry.