Rarity, while under the influence of Discord in The Return of Harmony Part 1, digs through a slab of rock which has three gems reminiscent of her cutie mark embedded in it. She breaks apart the rock and extracts a large diamond, acting fawning and protective towards it. When Twilight Sparkle arrives, the "diamond" appears as a simple boulder, making it clear that only Rarity sees it as a diamond. Rarity refuses to part with it for the majority of both episodes, and Twilight even helps Rarity carry it in hopes of eliciting a positive response from her.
The next episode, Twilight asks her friends into her library, but Rarity remains outside and says she's on to Twilight's plan to steal "Tom" from her, addressing the boulder by name. Resigned, Twilight carries the boulder inside for Rarity and crashes it through the door, leaving a giant hole in the wall. Later, as the other ponies venture back outside, Rarity delays in order to push "Tom" ahead of her. Twilight magically levitates the rock, then the scene cuts to outside the library, and Twilight is heard exclaiming "Look out, here comes Tom!" before the boulder flies out the window, leaving another giant hole in the wall.
Near the end of the episode Twilight reverts Rarity to her normal self, and the illusion that the boulder is a diamond goes away. She rolls it out of her house and asks her friends to never speak of it again.