|—Lighthouse Gem Monster, "Horror Club"|
She consists of a gigantic mouth grown on the wall with Lars' name on it.
She was trapped inside a wall in the Lighthouse.
When Lars and Ronaldo were young, Lars carved his name in one of the walls. In response, the gem loosened a plank from the wall and smacked Lars with it. Ronaldo took a photo of this event, intending to use it as evidence of the Lighthouse's possession, but Lars feared embarrassment and tore up the photo. The Gem recorded Lars' and Ronaldo's falling out.
Years later, Steven, Lars and Sadie went to the Lighthouse and watched a movie with Ronaldo. The power in the Lighthouse went out, likely due to the Gem. Ronaldo suspected a ghost and brought out a pendulum. The Gem sent the pendulum flying into Lars' face, then she lifted many folders from a filing cabinet which circled the humans. After throwing a few more objects at the group, she tried to trap them by forcing the only door shut, but they managed to charge through it. They attempted to exit the Lighthouse, but the Gem attacked them using scarecrows which were placed outside by Ronaldo earlier. The group forced out the scarecrows and shut the door, leaving themselves inside the Lighthouse.
As the group argued amongst themselves, the gem drew back some floorboards from under Sadie, causing her to fall in the basement, then she closed the floor. She opened a trapdoor for Steven, Lars and Ronaldo to enter the basement.
Inside the basement, the group saw the Gem's physical form, which grew a mouth right before their eyes. Ronaldo attempted to sacrifice Lars to the Gem by throwing him into the mouth, hoping to rescue Sadie. Steven jumped in to protect Lars; as the Gem tried to bite into the two, Steven formed his bubble shield and forced the Gem's teeth back, and this destroyed her mouth and physical form. Sadie was unharmed, and as she emerged from the mess, she pushed out the gemstone on to the floor. Steven picked up the Gem, and she immediately played back her recording of Lars and Ronaldo.
After watching the recording, Steven bubbled and stored the gemstone. Sadie and Steven understood that the Gem felt hurt by Lars' carving, and that her aggression came from being trapped for a long time.
- Possession: She has the ability to possess, although it is unknown if this ability is limited to inorganic or non-living matter.
- Host Body Manipulation: While possessing the lighthouse, she was able to manipulate different parts of the building, like forming flesh-like matter and a mouth on the basement wall.
- Partial Possession: She has many paranormal-like abilities, such as possession of inanimate objects like scarecrows and fake bats, meaning she is not limited to only the lighthouse, but nearby objects or items touching it.
- Telekinesis: She carries the power of telekinesis, which is shown when she was able to levitate and throw objects inside the lighthouse.
- Memory Projection: She has the ability to project recordings of past events she has witnessed, just like Pearl. This also bears a resemblance to Lapis Lazuli's photographic memory-like ability to replay past events as seen in the episode "Mirror Gem".
- Hologram Projections: The Lighthouse Gem Monster can project holograms like an uncorrupted Gem, with enough skill to create accurate "playbacks" of past events from memory.
|1. "Gem Glow":||Absent||14. "Lars and the Cool Kids":||Absent|
|2. "Laser Light Cannon":||Absent||15. "Onion Trade":||Absent|
|3. "Cheeseburger Backpack":||Absent||16. "Steven the Sword Fighter":||Absent|
|4. "Together Breakfast":||Absent||17. "Lion 2: The Movie":||Absent|
|5. "Frybo":||Absent||18. "Beach Party":||Absent|
|6. "Cat Fingers":||Absent||19. "Rose's Room":||Absent|
|7. "Bubble Buddies":||Absent||20. "Coach Steven":||Absent|
|8. "Serious Steven":||Absent||21. "Joking Victim":||Absent|
|9. "Tiger Millionaire":||Absent||22. "Steven and the Stevens":||Absent|
|10. "Steven's Lion":||Absent||23. "Monster Buddies":||Absent|
|11. "Arcade Mania":||Absent||24. "An Indirect Kiss":||Absent|
|12. "Giant Woman":||Absent||25. "Mirror Gem":||Absent|
|13. "So Many Birthdays":||Absent||26. "Ocean Gem":||Absent|
|27. "House Guest||Absent||40. "On the Run":||Absent|
|28. "Space Race":||Absent||41. "Horror Club":||Debut|
|29. "Secret Team":||Absent||42. "Winter Forecast":||Absent|
|30. "Island Adventure":||Absent||43. "Maximum Capacity":||Absent|
|31. "Keep Beach City Weird":||Absent||44. "Marble Madness":||Absent|
|32. "Fusion Cuisine":||Absent||45. "Rose's Scabbard":||Absent|
|33. "Garnet's Universe":||Absent||46. "Open Book":||Absent|
|34. "Watermelon Steven":||Absent||47. "Shirt Club":||Absent|
|35. "Lion 3: Straight to Video":||Absent||48. "Story for Steven":||Absent|
|36. "Warp Tour":||Absent||49. "The Message":||Absent|
|37. "Alone Together":||Absent||50. "Political Power":||Absent|
|38. "The Test":||Absent||51. "The Return":||Absent|
|39. "Future Vision":||Absent||52. "Jail Break":||Absent|
- She's capable of projecting recordings of events that she has witnessed, similar to Pearl's ability to project holograms, and Lapis Lazuli's ability to record events she has witnessed through the mirror she was trapped in. It is the first (and so far only) Gem monster to demonstrate this ability.
- She is the first Gem monster to possess the ability to talk. However, her speech is faint and disorientated.
- She is the first Gem monster to target a specific individual, with Lars being that individual.
- It's questionable if she is corrupted as she hasn't been seen using a form. However, the big mouth might have been her form, which would explain the mouth poofing.
|Lighthouse Gem Monster's gemstone doesn't have a known physical location.|