These are still dropping on eles, and I and I bet others have then stored away in chests from the old imbu system. I sugest taking them out of the P coin room, stop them dropping on eles and putting a stone for each like the big fish one where they can be exchanged for the new imbu mats.
Unless i'm mistaken i even think on some elementals they drop instead of gems like it's supposed to now.
I've killed several acid elementals, never got a gem but got several of those essences
It's all good :)