The Crystal Gems are searching for the monster at the beginning of "Island Adventure". Later on, the monster attacks Lars, Sadie, and Steven, causing Steven to summon his bubble to protect them. Sadie subsequently defeats the monster by stabbing her through the torso with a spear.
As her name suggests, the Invisible Gem Monster's body is invisible, except for her gemstone. Her features become visible when substances such as mud or water coat her exterior, revealing her to have a bulky body, strong build, two eyes, and a mouth. She doesn't appear to have a distinct head or neck. Located on her chest is a cone-shaped gemstone 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. The monster appears to prefer walking on all fours rather than upright, although it is unknown if the monster can actually walk on two legs or if she is disabled, damaged, etc.
Invisibility - The Invisible Gem Monster's primary ability is, as the name suggests, invisibility.
Stealth - With her invisibility powers, the monster is skillful at stealth, as the Crystal Gems thought she escaped from Mask Island.
Enhanced Strength - She's incredibly strong, with enough power to pop Steven's bubble shield. However, Steven panicked at the time his defenses were breached, so it may not have been the monster's strength as much as Steven's concentration being broken.
Invisible Gem Monster is the first gem known to have been poofed by a human. Sadie was able to poof it with only a sharpened stick. This proves that early humans could have posed some trouble for the colonizing forces from Home world.
Invisible Gem Monster's gem is the first shown example of a gem having more surface area hidden inside the body than exposed outside. Due to the gem's rotation it forms a spike pushing into the hard-light body.