Lexonite is a peculiar radioactive element that appeared the two-part episode, "WordGirl Makes a Mistake". In its solid, rocky form it is a bright red that pulses and fluctuates with a softly glowing light, like Dr. Two-Brains's mouse brain. It has a rough texture and comes, accordingly, from Lexicon, WordGirl's home planet.
In the episode "WordGirl Makes a Mistake," a large chunk of Lexonite falls to Earth, coincidentially during an encounter between WordGirl and Mr. Big. WordGirl immediately begins to feel the negative effects that the mineral has over her - it eliminates all of her supernatural superpowers (flying and the like), and even cuts on her immense vocabulary so she misuses and mispronounces words like "parsculiar". Later on in the episode the Lexonite is made into a star-shaped collar and into a large cage. It is unknown what happened to the large quantities afterward.
Interestingly, the Lexonite appears to have no effect on Captain Huggyface, possibly even insinuating he's not from the planet Lexicon. (Either that, or only humanoids from Lexicon, like Becky, can gain superpowers.)
The Lexonite returned in an episode called "Gift Pony", made from a brooch given by Violet to Becky.