The "Lighthouse Gem Monster" is a corrupted gem that first appeared in "Horror Club". It's notable for causing the lighthouse to appear possessed.
It consists of a gigantic mouth grown on the wall with Lars' name on it.
"Horror Club" It was trapped inside of the Lighthouse wall, until Steven, Lars, Ronaldo, and Sadie showed up to free it. Its gem was bubbled and safely stored by Steven.
- Possession: It 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, it was able to manipulate different parts of the building, like forming flesh-like matter and a mouth on the basement wall.
- Partial Possession: It has many paranormal-like abilities, such as possession of inanimate objects like scarecrows and fake bats, meaning it is not limited to only the light house but nearby objects or items touching it.
- Telekinesis: It carries the power of telekinesis, which is shown when it was able to levitate and throw objects inside the lighthouse.
- Memory Projection: It has the ability to project recordings of past events it 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.
- It's capable of projecting recordings of events that it has witnessed, similar to Pearl's ability to project holograms, and Lapis Lazuli's ability to records 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.
- It is the first Gem monster to possess the ability to talk. However, its speech is faint and disorientated.
- It is the first Gem monster to target a specific individual, with Lars being that individual.
- It's questionable if it is corrupted as it hasn't been seen using a form. However, the big mouth might have been its form, which would explain the mouth poofing.