If you have the cash in hand to pay for the ring, that sure beats a loan–then you only pay the cost of the ring, not cost of ring plus interest.
ETA: Though I’m still not sold on the issue of opportunity cost–what are you giving up the option to spend that bonus on if you spend most of it on a ring?
I wouldn’t be surprised if vintage Tiffany rings are (mostly) only going to increase in value. But I bet in the short term that people willing to pay 18K+ for a Tiffany diamond ring are going to go directly to the source. (Sorry, not going to research that for you). Plus, don’t forget the difference between what you pay the retailer for an item you buy, and what the retailer will pay you if you sell it).
But, really, even if you can afford $18K for a ring, and you are sure she’d love it, I think you need to talk to her about your finances, your priorities, and your anticipated income for the next few years, as well as the next twenty or so years. And the military is paying for school? Great, but have you and she thought about what that means for your/her lifestyle for the next few years? (Not being specific, because I’m not military, I don’t know what you are looking at. But if you will be serving in the military, even as a dentist rather than a high risk role, you need to think about what this means for your lifestyle.)
You know, I don’t want to be discouraging, and put a damper on your dreams. And if your girlfriend would be happiest with an expensive ring, and you can afford it, go for it, be happy, it’s your money.
But 18 K is a lot of money to spend on a diamond ring. Especially if you are young, about to graduate, in the military, etc. And if you are thinking of this ring as an investment, keep in mind that (even aside from the difference between the value of the ring as you buy it and the value of the ring if you sell it) selling the ring may not be an option available to you. (In other words, you break up or divorce, she may keep the ring.)