These were designed to be able to have easily changable "gems" to match whatever outfit she may want to wear, but it would be easy enough to take this and make any sort of Lego earring style from it.
Step 1: Stuff You'll Need
Earring loops (or some other small ring)
Lego blocks depending on what you want the earring to look like
I bought a small variety back of loops for a few other projects. The size of loop you'll want will depend on the looks you want and the size of the block you're connecting to. I used the 1/8" loops for the earrings here.
Step 2: Glue the Backs
Also, if it's important to you, double check to make sure that "Lego" is right side up on the top of the studs.
Step 3: Add Earring Back
Once open, slip it through the hole in the loop attached to the Lego block.
Then, carefully reclose the loop in the earring back.
Step 4: Add Flair
Then, surprise your girlfriend and go to your favorite Lego convention. Or just a night on the town.