Gem-Eels are jewel-encrusted eels that are composed of thousands of brightly colored gems. They appeared in Gill Grunt and the Curse of the Fish Master. While safe underwater, the gem-eels become heaps of precious gems when on dry land.
The gem-eels were a target for Captain Grimslobber, who planned on caging the poor sea creatures and hauling them on land to make himself rich. As the gem-eels were immune to the power of the Fish Master's Crown, Grimslobber hypnotized the mermaids to dig them out for him and put the eels in cages. The eels were eventually freed by the Skylanders.