Mask Island is an island that first appeared in the episode "Island Adventure".
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 and ocean fish with strange mask-like faces and glowing eyes.
"Island Adventure" and "Marble Madness"
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 and build a hut to live in. After fighting the Invisible Gem Monster, they leave.
It has a brief appearance in Marble Madness when one of Peridot's Plug Robonoids crash-landed on the island while trying to locate the Prime Kindergarten Control Room. The Crystal Gems warp there to fight it.
At some point after this, the Watermelon Stevens arrive on the island and develop a small civilization. They advance remarkably quickly and appear to have knowledge of the wheel, basic metallurgy, ballistics and even gliders.
During the final hours before the drilling operation to confront The Cluster, Steven inadvertently controls the body of a Watermelon Steven while sleeping. He explores the island and befriends a watermelon dog. During his exploration on the watermelon village, he is selected by the village elder to stand upon a cliff overlooking the ocean with a flower crown. Steven does so, only to be attacked by Malachite, sending him back to his waking body. He urges the crystal gems to go to Mask Island and fight Malachite, stating that she is causing the Earthquakes. The Crystal Gems depart via Warp Pad, appearing on the island and quickly forming Alexandrite to combat the rogue fusion. During the battle, Steven assumes control of another Watermelon Steven and encourages the civilization to come aid Alexandrite against Malachite. The villagers attack, armed with spears and catapults, distracting Malachite long enough for Alexandrite to gain the upper hand in the conflict. After a long-winded battle, Alexandrite manages to split Malachite back into Jasper and Lapis, catching the two as defeated and unconscious gems. Exhausted, Alexandrite splits back into Garnet, Amethyst, and Pearl upon reaching the shore.
After Malachite is defeated, earthquakes begin rocking the island and creating fissures in the ground, threatening to swallow Jasper and Lapis. The Crystal Gems manage to save Lapis but lose Jasper as she slides into a fissure and vanishes from sight. Garnet realizes The Cluster must be taking form, but the island's warp pad was already destroyed, leaving the Crystal Gems with no quick way to return to the barn. Finding Steven among the gathered Watermelons, Garnet orders Steven to wake up and return to his true body to help Peridot stop The Cluster. While Steven and Peridot begin drilling through the Earth towards The Cluster's location, The Crystal Gems retreat from the island with Lapis. It is currently unknown how much damage was done to the island from the earthquakes, or if the Watermelon Stevens survived.
- 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 cooked, these expressions become sad, similar to a tragedy mask.
- Garnet revealed the island's name in the episode "Marble Madness".
- While Mask Island's warp pad was destroyed in "Super Watermelon Island", the Crystal Gems are later seen warping back in "Gem Drill", so there seems to be another nearby warp pad within several hours of the island.
- The Watermelon civilization has an intriguing mixture of technology. They possess at least basic metallurgy, judging from their gong. Their gliders bear a resemblance to the Ornithopter.