Then buy the ring! A think it's a very nice gesture to include the parents and it sets a good foundation for the future. I wouldn't include them that much in advance. I would only let that conversation air out for no later then a week. Don't put her mom in state of excitement and have to wait long. Also, have you considered going the non diamond route or even considered synthetic gems? One more thought by the way, have you ever talked marriage to your girlfriend? Is she on board for marriage? Have you talked out money, religion and how to handle other life issues? Prepping for marriage should be somewhat unsexy and very blunt. Don't take lightly the gravity of what you are about to enter. The movies make you think that you buy the ring and have a wedding and everything just works out. That's bullshit, the act of marriage is tough and long and if you haven't had the foundation talks with your partner you are not prepared for the future.