Regal Diamond

The Regal Diamond is a treasure in Pikmin 2. It is a clear, brilliant-cut. 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.