Girl here. The comments you've gotten so far are going about this the wrong way. Arguments about price, ethics, practical tradeoffs are the dead wrong tone.

Girl here. The comments you've gotten so far are going about this the wrong way. Arguments about price, ethics, practical tradeoffs are the dead wrong tone. Proposals should be over the top romantic; you'd never suggest skipping a beautiful or dramatic location to save money, right? Beyond that, this isn't just about how she feels about it. Everyone she knows, relatives, friends, is going to want to see the ring and hear the story. To all of them, a diamond is the gold standard and a cheap substitute will be an awkward counterfeit. Even if you convince her, you are signing her up to have the ultimate celebration of her, the once in a lifetime birthday party, turn into a lot of awkwardness and sympathy. Bad deal. Ignore the cost. It's impolite to ask and moissanite is expensive enough. If what you do looks legitimate enough, no one need know. The critical key is that you sell this as better, representative of more thought, more effort, more investment on your part. Focus on that you're doing something different. Something special. Design a custom ring that speaks to you about who she is. Focus on the ways in which the stone is superior to a diamond. It is unusually brilliant which reminds you of her. It is tougher, harder than what everyone else takes for granted. You mounted it with blue stones for her eyes. Make it personal and in every possible way, about her and as much as possible tell a story that will make her friends wish someone that thoughtful cared about them enough to do something truly special rather than checking the expected box.