Tom

Tom is the given name of Rarity's rock in the episodes The Return of Harmony Part 1 and The Return of Harmony Part 2. Tom was shown at first to be a huge diamond (referred to by Rarity as a "handsome hunk of a diamond"), but later turned into a simple boulder.

Tom was used by Discord to tempt Rarity into losing harmony with her friends, and turning against them. Rarity seems oblivious to Tom's status as a boulder, not a diamond, probably due to Discord's magic.