sfumato, otherwise knows as "leonardo's smoke", is a shadowy quality that leonardo da vinci used quite frequently is his completed works. it is, in fact, this exact type of quality used around the mouth of the woman depicted in his small portrait of mona lisa that keeps people guessing as to whether she is really smiling or not, and if so, what type of smile is that? :)
cool question. leonardo da vinci was an amazing individual. pure genius.