Hi Danielle:
I'm going to hope that you shut the water off as soon as that earring hit the drain and perhaps it hasn't gone too far!
If you are lucky the earring is stuck in the
tub waste assembly (see pic.). It could also be stuck in the
PTRAP just under the tub waste.
First thing to do is to get to the ptrap and see if you can remove the
cleanout at the bottom (see picture...unscrews clockwise). Get a bucket and unscrew the cleanout and see what falls into the bucket (you can be sure water from the trap will fall into the bucket). If no earring falls into the bucket at that point then you will need to run the water in the tub and see if you can force the earring into the bucket. Jiggle the trip lever a few times in case it got stuck in some soap scum...
Try these things...maybe you'll get lucky!
MARK