underwater materia

guide book
earth harp
or the desert rose

guide book is in the underwater reactor but you have to morph a thing called a ghost ship to get the item. It’s in the tunnels, walk around long enough and you will fight one. Take it back to the Kalm traveler and he will give you the underwater materia.

Earth Harp and the Desert Rose you get from the two WEAPONS. Earth harp from emerald WEAPON and Desert Rose from Ruby WEAPON take them back to the traveler to get the following:
master summons
master magic
and master command

for the desert rose you get a gold chocobo

you only need the guide book but the other items are worth the prizes.