From Pikipedia, the Pikmin wiki
The Regal Diamond (ロイヤル・ダイヤモンド?, lit.: "Royal Diamond") is a treasure in Pikmin 2. It is a clear, fashioned diamond. The jewel is found on the last (fifth) sublevel of the Citadel of Spiders, and is completely buried in the ground. Go to the area with some Glowcaps and an Iridescent Flint Beetle and toss White Pikmin onto the ledge in which the treasure is buried, which can be identified using the Treasure Gauge. They'll dig it up, and there should be no hassle transporting it back to the Ship's Pod.
“The other day, my daughter begged me to get her some jewelry. She's becoming so fashion conscious! I know girls grow up quickly, but never imagined it would happen this fast!”
“This scintillating jewel will make girlfriends feel like queens of the galaxy and boyfriends feel like kings of debt. Fit for the ruler of the universe, it is the ultimate giant diamond.”
Names in other languages