What do i do if i dont have the money to buy her the ring that she wants?

I want to propose to my girlfriend, but i dont have the money to get a very nice ring. My mom gave me her engagement ring that my father used to propose to her becuase they are married. I was going to take it the the jeweler and have them resize it and shine it up. However, the ring my mom gave me to use is gold, and my girl friend said she wants silver. What do I do?