What can I do with Treasure? [duplicate]
Yes.
Some treasure items are required to upgrade certain weapons you find through the game (denoted by a yellow "+" symbol next to them). Additionally, you may use some treasures as tertiary spell components and significantly boost spell potency, duration, range, or provide a secondary effect that can either help the caster or assail the target with a negative status on top of damage.
Treasure items serve three purposes
Item Upgrades
Certain treasure items are used in item upgrades, but this is actually a pretty short list, at least relative to the number of treasure items in the game.
- Rusted Bit (Engine Blade 2)
- Glass Gemstone (Engine Blade 3)
- Sturdy Helixhorn (Ultima Blade)
- Metal Scrap (Drain Lance 2)
- Coeurl Whisker (Drain Lance 3)
- Spiked Armor (Absorb Shield 2)
- Barbed Scythe (Plunderers 2)
- Earth Gemstone (Valiant 2)
- Monster Claw (Force Stealer 2) - Cactuar Needle (Auto Crossbow Plus)
- Dynamo (Bioblaster Plus)
- Sky Gemstone (Circular Saw Plus)
- Hydraulic Cylinder (Gravity Well Plus)
- Magnetron (Noise Blaster Plus)
- Magitek Core (Drillbreaker Plus)
Elemenacy
When creating spells, you can add an item (Medicine, Ingredient, or Treasure) to them. Any treasure can be used for Elemancy, with varying effects.
Selling
As mentioned in the question, treasures can also be sold for gil. In the postgame content, this may actually make up a non-negligible portion of your income, depending on how you play.