Rarities

To upgrade Equipment rarity, you must use Dust. The Dust cost changes as you upgrade, higher rarity = higher cost.
When a weapons' rarity is upgraded, the base damage increases!

You can also upgrade items, and armor. Most items and armor are upgraded by fusing three of the same rarity together to get a higher rarity.
Armor upgrades with Gem Fragments (📙Upgrading) until it hits Unobtanium, at which point it also requires Dust.