The severed, stretched and dried skin of your girlfriend.
Rip out her large and small intestine, make a single lateral cut down the entire length, then cut it into 6" x 6" (180mm x 180mm if you use the metric system) squares, then stitch them into a patchwork jumpsuit.
Put her through a meat grinder, and slather her remains all over yourself. Then pour a bucket of liquid latex over yourself to make yourself look like you are wearing a giant condom.
Have her cremated, or do it yourself, and send the ashes to that place that will compress her ashes into a synthetic diamond. You can have it set in a necklace or whatever jewelry and wear it. There would be an added bonus of being able to some day have that gem set into an engagement ring when you find that special someone.
...for some reason, I don't think she'll pick one from this post. Even though i wrote it, i would advise against even considering anything I wrote.
If you do wind up picking one, please post some pics of the result.