Iridescence: Increases the bonus and penalty of weather effects by an additional 10%.
A small test that I did in abyssea la theine:
atmas:
rescuer: 10% cure pot
harmony: 10% cure pot (Really gave me 1%, however I was missing phalania locket to cap w/o. Replace this with minikin)
allure: 30mnd -30enm
Stats:
mnd: 199
vit: 115
healing skill: 454 ( I have healing magic merits)
cure pot: 50
Arka IV:
cure IV: 1018
rapture Cure IV: 1624
Chatoyant:
Cure IV: 1102
rapture Cure IV: 1760
Basically any situation where you can cap cure pot or get close to it while using chatoyant staff it beats arka IV.