This book was probably the first in a series, or was intended to be. The plot as I recall it was this stone was found by a girl and she wished on it that she was pretty like her older sister was (they were the YA classic duo of “the pretty one and the smart one”) but instead of the stone changing her with obvious magic, instead the MC woke up the next morning and had a sudden drive to change her appearance- working out, getting a better haircut, using cosmetics, getting her parents to get her contact lenses, etc. However, after a while of caring about her appearance, the MC notices that she’s become kind of a bitch- I specifically remember her watching some TV program (a talk show maybe) and completely ignoring whatever was going on to critique the host’s “thunder thighs.” And once the MC realizes that it’s the stone that’s making her think this way, she gets rid of it but it’s hard because it has some sort of psychic hold on her. By the end she comes to a middle ground with the whole appearance thing, going back to her glasses but keeping the cosmetics and still working out sometimes but not obsessive like she was under the stone’s influence. It’s heavily implied that someone else is going to find the stone which is what makes me think it was a series.

Other random bits I remember that may help:

-I read this somewhere between late elementary to early middle school, approximately 97-2002

-the stone itself was repeatedly described as looking like “a 3-d Valentine” and I think came from space? it might have been a meteorite

-the sister was specifically written as really nice but the MC had a really hard time accepting the niceness as genuine

-I think the cover was purple?

Hope this rings a bell with someone!