My fiance chose an aquamarine - it's perfect. Slightly non-traditional, but still beautiful. Also, they don't generally set aquamarines in "engagement" settings, so he ended up getting me a custom ring (for far, far less than he would've spent on an "off-the-rack" diamond ring). I think you should have something you love, and not worry about what's traditional.
Also, you mentioned you love purple - have you considered an amethyst? They're beautiful too, and probably more easily found than a truly purple pearl.