I don't know if this is still the case - but I found it best to farm on white or, hence, even blue named mobs. Especially if you need some gear, before you level too far - these blue named mobs give the lowest exp gain, so you can farm enough of them before leveling.
Although, with these ridiculous low exp gains all over the board - it doesn't really matter, anyway.
Generally:
Farm = blue / white
Level = orange / purple (if you can kill them fast enough = aka enough gear), unlike most think.. it's rather redundant to go fight purple named mobs if you take too long to kill them..
Grey = no drop, no crones, no exp
It could always just be that it won't drop due to the extremely low drop rates.. but you should get crones, at the least.