Greens and rares do drop (at least from 80+ mobs, i believe 79- have worse rate), however only one every around 10 mobs (during drop rate event). Considering their HP and people being accustomed to the drop rate of previous mobs, its a horrible task to farm these for stones. Also decomposing rares gives the same as decomposing uncommons => 2 stones each.
Regardless, they do drop (but +10ing my set will probably take several weeks longer...).