So far, the birthstone has served no purpose at all. The attribute you choose when you create a character (Strength, Wisdom, Liberty, ectera) supposedly determines where you get dumped at on Birthday Island. Not sure if that's true or not though.
From MMORPG's article on the upcoming release of ROSE (found at:
http://www.mmorpg.com/gamelist.cfm?setview=features&loadFeature=293&gameID=183&fp=1024,768,1127018343,20051112015849 ):
Appearance is semi-customizable with hair, face and birthstone choices (birthstones are used as your cursor throughout the game).
So maybe they'll look nice. ;P