The Adamantine Girdle is a treasure in Pikmin 2 that is actually a shiny nut. It can be found on the 4th or 5th sublevel of the Subterranean Complex, depending on the game version. It is the 99th treasure on the List of Pikmin 2 treasures in the NTSC version and the 98th in the PAL version.
Captain Olimar's Journal
"There appears to be a link between the treasures we found in an underground structure. We found several machine parts that fit together. The parts may provide a clue as to the original function of the area."
"None want to believe it: a world so brutal as to require a steel torso guard for self-defense. It is certainly unattractive, but which do you value more: fashion or personal safety?"