Unusual is a quality for equipment that has a special particle effect. Couriers are the only type of equipment that can be Unusual, and can only be obtained at a 1–2% chance through opening certain Treasures.
Unusual Couriers may have one of many different effects: a flurry of snow, bright beams of light, burning fire, or more. The effect is specified by which Ethereal Gem is placed inside it. An Unusual courier can be given a new effect by socketing another Ethereal Gem inside it, overwriting the old one. NOTE: in reborn you can't do this socketing or overwriting the old gem.
Each effect can be one of many different colors, specified by a Prismatic Gem. Like Ethereal Gems, the color of an Unusual Courier can be changed by inserting a new Prismatic Gem.
Unusuals come with one Ethereal Gem in an Ethereal socket and one Prismatic Gem in a Prismatic socket. Inserting a new Ethereal or Prismatic Gem will overwrite the current one. Ethereal and Prismatic gems can be extracted with a Master Artificer's Hammer and placed on a different Unusual courier. Using a Master Artificer's Hammer will destroy the original courier.