aetheral items

[-DMR-]

In the armory chest I have a couple or so items that are marked as 'aetheral' and have a shaded colour background. However, I can't obtain the items until the character is Lv24 in the class they're for.

[zemogFC]

Those pink and green gears are items you've greeded or needed on in dungeon chests.

They could be for any class, so you may never use them at all. Just save them on your retainers for now. You'll be able to exchange them for Grand Company seals later in the game.

[Jeannes]

If they're in your armory chest I don't know how you can't obtain them when you already have them.
If they are not your level or class then you obviously cannot equip them.
Aetheral and Unique items are just buffed equips mainly obtained as dungeon loot.

[Niwashi]

Judging by your sig, it looks like you've at least started most of the classes, so if that means you plan on leveling the rest eventually, you might want to hold on to those aetherial items until the appropriate class is high enough to use them, that is, unless you also got a corresponding green-background piece that's even better for them.

Gear quality while leveling up is basically
(1) (pale grey background) standard gear like you get from quests, NPC vendors, or normal-quality crafting results.
(2) (pale grey background with sparkles) High-Quality results from crafting
(3) (pink background) aetherial gear dropped from dungeons (actually, this is a tie with High Quality gear that has materia added)
(4) (green background) unique dungeon dropped gear sets.

Once you get to endgame, there's more quality levels after that. (I think it's the blue background that comes next.) But by then, you'll mostly be looking at their Item Level.

If you don't plan on leveling the associated class, pink and green gear can be traded to your GC for a few GC seals. (And some green gear can be sold on the marketboard, but most cannot. I don't think any of the aetherial pink gear can be sold on the MB.)

[Urthdigger]

One other thing to note: All aetherial gear are modified versions of HQ crafted gear. So, an aetherial sphene ring would be an HQ sphene ring with some bonus stats. This is mostly important as it means that, like normal crafted gear, you can spiritbind it to make materia. Likewise, not all green items are dungeon drops. Plenty of them are crafted "vintage" items, which are crafted using "damaged" armor dropped in dungeons. Like other green gear, they have a higher item level than their equip level, so they're stronger than other items, but like crafted items they can be spiritbound. Likewise, there green crafted items that are over level 50 crafted using materials from high level gatherers and materials bought with tomestones.