I'm in the same situation! My dad's mother passed away when I was younger and now that I'm 22, I was given her engagement ring. It's a 1 carat ring with much smaller stones in designs on the side and it's set in platinum. I absolutely love the ring as-is, but I don't want to wear it on my left ring finger (my boyfriend and I have been dating for 6 years...I don't want to answer the questions that would come). I've been thinking about having it sized to wear on my right ring finger. I would love to use it as my engagement ring one day, but my boyfriend feels strongly about purchasing a ring on his own when the time is right. Such a dilemma, but I hate to have it sit in my jewelry box when it's such a beautiful and sentimental piece.
Whichever option you choose, I'm sure it will be beautiful and special to you. Good luck and be sure to let us know what you decide!
