Okay here's my take. The ring is probably platinum, not silver because I thought they don't set and never have set diamonds in silver. A jeweler told me this years ago. However, give us some details. Depending on the size of the stone and the size of her hands it might be too big or too small for her. That aside, I don't think giving an heirloom ring is a bad thing. I do, however, feel like if you got the ring at no cost that you could buy her some other jewelry like diamond stud earrings. This will let her know that though the ring was in the family you would have gone out and bought her a ring like a traditional situation. This is a hard sentiment to translate into writing but I hope you get it.
When you can, tell us more about the ring.
OOOOHHH. I just had a thought. Propose to her with the ring and explain to her it's a family heirloom and let her know she doesn't have to keep that ring if she would rather have something you both pick together that is okay. After all, she does have to wear the ring. My mother gave me a .25 carat ring (her engagement ring) and when I got engaged and he put that 1 carat on my finger I thought, "I used to love my mother's ring but wow, this one is all mine" not to mention much bigger.
My grammatic skills suck!