Open a chest near the beginning of a chapter. If it does not contain a
diamond or other desired crystal, do not press X, as it will trigger the
game to auto-save. Instead, hold the PS button, and restart the game.
You will start at the beginning of the chapter, and the chest you had
just opened will be closed again. Open the chest again. If you do not
get a diamond or other desired crystal again, repeat the process until
you get a diamond or other desired crystal.