Jewelry stores offer attractive financing, but while I don't recomend marriage or engagement untill after graduation when your stock rises, I have the following insight.
I worked as at a ritzy jewelry store in college and most stores of these types have cz stones in all their settings and then sell the diamond additionally. Take the old lady out and look at settings and then supprise her with her choice and never discuss the economics. Also never discuss the stone, chances are she'll just love it, after all, cz are much more spectacular than the real thing.
Then if you still love her when you get your first big check "upgrade" her diamond with or without her knowlege. She'll never know what the first stone was and if she's the type that would have it appraised 😱
RUN MAN RUN.