Princess Pearl

The Princess Pearl is a treasure found in Pikmin 2 and Pikmin 4. In Pikmin 2 it appears as something resembling the bead of a, an earring, or a Christmas tree ornament. In Pikmin 4 the treasure instead resembles the natural pearl of a clam. Being found inside of a Pearly Clamclamp it can be assumed that it is the creature's pearl, though serving the role of a treasure in this game as opposed to the creature's carriable corpse in.

Collecting the treasure in Pikmin 2
The item can be found on sublevel 6 of the Cavern of Chaos, inside a Wollyhop.

Collecting the treasure in Pikmin 4
It is inside of a Pearly Clamclamp in the circular arena at the northeastern corner of the Serene Shores. It is much easier to attack when the tide is low, as otherwise the arena is fully submerged. To collect this treasure, you must first defeat the Pearly Clamclamp, then throw 3 Pikmin at it.