This seems to be the fastest way to Max out Gems, It's Very Late in the game. I Recommend watching a walkthrough for this area of the game.
This Video is taken place after completing this Dungeon.
Once you reach Mermaid Cliffs
You'll keep to the right and you will see a Dungeon. If your on par with the Story at this point, You'll get the Plastic Explosive from the Zombies at the far right end of Mermaid Cliffs
and you'll head back to the Dungeon and use your level 2 Storm Puff power to explode the door to get in.
Once you reach Hypno Baron's Dungeon, You'll Traverse Through Until You reach this area...
(Elephant Dance Required)
Going through this area you will eventually reach the save point, SAVE HERE. Carrying on with the game you will go through a series of puzzles to obtain the keys you need. You need the keys to progress. (4)
from the Save Room
, Go in the Left door
Ride up using the eyeball platform
Make sure the LEFT EYE is looking UP
and the RIGHT EYE is looking DOWN
then enter the Room and Grab the gems
Then Make the EYES LOOK AT EACH OTHER
Then Enter, Go through the door on the right each time after collecting gems
Then go all the way back, Drop Down, Back to Save Point and SAVE, Exit Game, load back up