Are treasures used as crafting components?
There are, in fact, certain treasures that are used in quests or to upgrade weapons (also considered quests).
I found a list of all of these "Don't sell" treasures in the game here:
- Barbed Scythe
- Cactuar Needle
- Coeurl Whiskers
- Cylinder
- Dynamo
- Earth Gemstone
- Glass Gemstone
- Magitek Core
- Magnetron
- Metal Scrap
- Monster Claw
- Repair Kit
- Rusted Bits
- Sky Gemstone
- Spiked Armor
- Sturdy Helixhorn
Thus, it looks like all other treasures are able to be sold without worry.
Any treasures, cooking reagents and medicine can be used to add effect to spells during spells creation (such as different debuffs, up to 5x chained spells, benefits like experience etc). Each of many treasures provides certain effect. More of that, the more expensive treasure is - the more powerful effect it will add to spell. In this terms - there are no useless items in game.