Mask Island

"Mask Island" is an island that first appeared in the episode "Island Adventure".

Description
The island consists of a giant waterfall, multiple lakes, a beach, many trees, and multiple crystal clusters and geodes. It is inhabited by magical creatures such as worms (living on the island itself) and fish (living in the ocean around it) with unusual mask-like faces and glowing eyes.

The Invisible Gem Monster inhabited the island before it was poofed by Sadie with a spear, and bubbled by Steven.

"Island Adventure"
Steven, Lars, and Sadie were trapped on the island after Sadie hid the island's warp pad to prevent Lars from leaving. They quickly became familiar with the land, being able to cook and catch fish, as well as build a hut to live in.

"Marble Madness"
It has a brief appearance when one of Peridot's Plug Robonoids crash landed on the island while trying to locate the Prime Kindergarten Control Room.

Trivia

 * Everything living on or nearby the island has a constant happy expression on their face, akin to a comedy mask - which could explain the name of the island itself.
 * When they are grilling the fish they caught, the expression on the fish's face turns into a sad one, akin to a tragedy mask instead.
 * Garnet revealed the island's name in the episode "Marble Madness".

Isla Máscara