Darkonium is found in the blue chests that are scattered around the islands. (These are in the order I found them)

1. Chest on Xeroph Isle at the bottom of the "bowl".


2. Chest at the end of the First Shrine on Xeroph Isle (before the "final" door).


3. Chest in plank maze on Palaish Isle.


4. Chest is inside a tunnel concealed below the plank maze on the path towards the shrine.


5. Chest at the end of the Second Shrine on Palaish Isle.


6. Chest on Infern Isle at the end of the spiral path on the north side of the island.


7. Chest in the Ruins (near the beginning) on Celeste Isle.


8. Clear the C rank Tourney at the bottom of the Xeroph Isle "bowl".


9. Chest on small island off of Infant Isle.


10. Chest on small island off Celeste Isle, end of 3rd shrine, before boss.


11. Receive from question Guy on Infant Isle, after 6th or 7th question is answered.


12. Chest at the top of one of the columns in the jungle of Fert Island.


13. Chest located next to the shrine door on Fert Island.