What if she wants a really big ring you can't afford?

My girlfriend and I have been dating for over four years now. She just graduated from college and I’m still in college until next year. We have talked about marriage and we know that we want to take that step when it is financially feasible. She has on numerous occasions given hints that she wants to get married soon after I graduate. However, she’s also directly said that she won’t accept a ring below 2 carats. She also told me that specifically it must be a 2 carat center diamond, not a combined carat weight of 2.0. We’ve had this conversation a couple of times over the years. Her number actually started out at 2.5 because that’s the size of her mother’s diamond. The only thing is her father was a professional NBA player who made a lot of money. Over the years the size she wanted went down to 2.0. Recently, she said she really loved me and wanted to marry me and would accept a 1.5 ring, but she didn’t seem very genuine. I’m not cheap and really want to make her happy, buy even when I graduate from college with a degree in engineering it will take me a while to afford such a ring. Even 2.0 carat diamonds online cost around 10,000 dollars by themselves. I’ve also hinted at some happily married couples I know that upgraded their rings on major anniversaries and such, but it seems like she wants something big and just keep that with no upgrades later on. Do you think I should wait and save up for this ring? Do you think she would say yes or be happy with a smaller ring? I really want her to be happy more than anything, but I’m on track to graduate with no college debt and I really don’t want to go into debt over an engagement ring. Thanks for the help in advance.