The ADAMANT isn't very interesting in its raw form, but it can be forged into the mighty Xcalber, second only to the Masmune in power. If you find yourself leaving the Floating Castle before defeating TIAMAT, but after you've found the ADAMANT, it may be worth a detour to get the Xcalber before returning.
The Adamantite isn't very interesting in its raw form, but it can be forged into the mighty Excalibur, second only to the Masamune in power. If you find yourself leaving the Flying Fortress before defeating Tiamat, but after you've found the Adamantite, it may be worth a detour to get the Excalibur before returning.
AvailabilityChapter 11 (Available after completing Mission 07)
Adamantite is an annoying component, because it cannot be found anywhere, only purchased. While there are more expensive components out there, you can find those, making Adamantite the biggest money sink in the game. Fortunately, only three weapons require it for upgrading, though the accessories that use it are pretty nice in their own right.