Invisible Gem Monster

The Invisible Gem Monster (possibly Chalcedony appeared in "Island Adventure." Sadie destroyed its physical body and it now resides within its gem.

History
The Crystal Gems were searching for the monster at the beginning of "Island Adventure". When the monster attacked Lars, Sadie, and Steven, Steven summoned his bubble shield to protect them. Sadie subsequently defeated the monster by stabbing it with a spear.

Appearance
The Invisible Gem Monster's body is invisible, except for its gem. Its features become discernible when substances such as mud and water coat its exterior, revealing it to be bulky with a strong build, two eyes, and a mouth. Located on its chest is a cone-shaped gem with a slightly elevated base. While the cone portion is a consistent dark blue, the base is separated by an elevation and contains differing shades of blue.

Abilities
The Invisible Gem Monster's primary ability is, as the name suggests, its invisibility. It is also incredibly strong, with enough strength to pop Steven's bubble shield. The gem monster may have some control over the weather: upon encountering Lars, Sadie, and Steven and knocking them off a cliff, a heavy downpour overshadowed bright daylight in a matter of seconds.

Trivia

 * This gem could be Chalcedony because of Chalcedony's ability to be dyed (as if with its environment) and its natural forming color matches that of the Invisible Gem Monster's gem.
 * This is the first gem monster in the series to be defeated by a regular human, without the direct intervention of the Crystal Gems.
 * The Invisible Gem Monster has the same gem placement as Amethyst, upon its chest.
 * Despite having hands, the Invisible Gem Monster runs on all fours.
 * So far, this is the only known gem that has the power to become invisible.