Serendipity Stone are items that can be used to reroll a stat on a costume or undergarments. They are purchased from Mirage Isle for 1 Login Badge, or bought in their wrapped form off of the Auction House.
A player wishing to reroll a stat simply right clicks on the Serendipity Stone, clicks on the costume/undergarment of their choices, and selects a stat they wish to reroll.
Note: Rerolling a stat costs 50 Labor Points. The costume or undergarment must also be of the Arcane grade or higher for it to be able to reroll.