"...teardrops of a maiden fairy feeling neither magical nor merry."
When they reach the Fairy Kingdom, they are in luck and happen upon a fairy in distress. Trix is a fairy from the Fairy Kingdom, up for trial for using magic on another fairy. She was being teased for her size and turned the other fairy's wings into prune leaves. Alex and Conner collect one of her tears, but also defend her case against the Fairy Council.
When the Evil Queen has the twins captured and takes their items for the Wishing Spell herself, Conner kicks the vial out of her hand in hopes of sabotaging her. She manages to activate the spell anyway by using one of Alex' tears.
It does not say if the tears have any other magical qualities.