I think the calligraphy style (typically Vine Monogram font) are the prettiest. Also, if you're really thinking of investing in one, and you know she'll wear it, I personally would go with a metal one. Gold or silver. It looks classier, to me, than the acrylic ones.
I have only seen the metal ones around here (at a big southern school) on necklaces. The acrylic ones I have seen are on key chains or back packs pulls.
Hybrid Mode
