Xyloks are a sapient species of crystalline lifeforms which apparently can be found inside asteroids. They can live for millions of years and are able to directly interface with computers and other forms of technology.
About 60 million years ago, an asteroid crashed on Earth, containing several members of this species, which became trapped in the planet's crust with no hope of escaping. They remained in this state for several ages until the Krakatoa eruption of 1883, in which one of the Xyloks was released. Pretending to be a benevolent entity, the Xylok ended up in Sarah Jane Smith's attic and started communicating with her technology and formed its plan to design Mr. Smith, (which is completely different from the Crystal, due to Mr. Smith being a Xylok computer in the wall). Mr. Smith's actual plan was to manipulate events to cause the destruction of the Earth and thus free his colleagues who were still trapped underground.
- Xyloks were featured in the TV Series The Sarah Jane Adventures. It was shown in the last episode of the series, "The Lost Boy", that Mr. Smith (Sarah's helpful computer) was a Xylok and was planning to kill Luke and destroy the Earth. Mr. Smith captured Clyde inside himself. Mr. Smith was evil and the Slitheen family were also trying to kill Luke. However, the child of the Slitheen said, "Never trust a Xylok. They're not on anybody's side, only their selves". Sarah Jane let K-9 (the robotic dog-like former companion of the Doctor) out of an explosion to wipe Mr. Smith's memory and give him a new purpose, 'To help save the Earth'.