From Diablo Wiki
This is NOT by FAR the HARDEST item to get.. i got this one on the first try :) the Liquid Rainbow IS the HARDEST to get !! ive been trying to get it for about an hour now !!
Droprate/bounty & exploration mode?
I have been after this item for a while now, and a while ago I found the correct cave, got to level 2 in the cave and also found the rare lacuni that drops the gem. In the article though, it says that it "WILL" drop if the lacuni spawns and you kill it - but this was not the case for me. I killed the correct mob - but it didn't drop the Gibbering Gemstone. Perhaps it is different if you are in bounties & exploration mode? Anywho - just wanted to add information that it CAN drop - but it doesn't always drop as the article says.