The Mermaid Eye is a gem that was found in the North Atlantic Ocean. It was only shown in Episode 7.
The gem is green, and it has many facets. Its structure is like a green crystal. When on display, the gem spun around. As it spun around the gem sparkled. It originally belongs to Rina Toin as it was seen sitting above Rina's shell throne. After Gaito attacked the North Atlantic Ocean Castle, the gem was lost in the ruins until some people found it in the Atlantic Ocean and brought it on display on a cruise ship. It was stolen by Eriru to lure the princesses out since the Mermaid Princesses would have some interest in the gem. After Eriru lost to the Mermaid Princesses' song, the gem fell to a deep part of the ocean and it was never found again.