You may already be doing this, can't tell from your post, but I wanted to say what I often say - consider proposing without a ring so you can go together to choose it. I think it's so much more modern and she can decide based on HER preferences to get what she likes best.
(For instance, I really hate gold and prefer platinum to white gold. I talked to a woman who said she wanted bigger diamonds so she compromised on the metal [white gold] but I felt more strongly about the metal than the diamonds. Everyone has their preference and I was very relieved my [now] husband gave me the choice.)
Unless you have some crazy elaborate plan for proposal, I really like the realistic approach, where you can tell her how much you can afford to spend on it (because finances are going to come up in the marriage, so if this turns her off, that could be a red flag there) and then she can pick her ring accordingly.
Just a thought.
Can't help with the diamond sourcing but it's a good idea to check that you're getting responsibly sourced diamonds rather than "blood diamonds".