I've been lurking for a bit, but this is my first post.
I'm not engaged yet, but from conversations with my bf I know he's planning on asking me SOON, like probably within the next couple weeks. Today, I sat down at the computer, and he hadn't logged out, and before I could look away, I saw thumbnails of two rings in his email. I'm NOT proud of this AT ALL -- we both really pride ourselves on giving eachother e-privacy -- but curiosity got the better of me and my eyes lingered longer than they should have. I read a few lines and then felt ashamed and logged out at that point.
In that short time, I gathered that the pics I was seeing was of a ring that was given to him, probably by a family member, but he had made an alteration to it (and I definitely read the word expensive). Initially, it was a three diamond setting, and he had swapped the center diamond for a sapphire.
So, here is the problem. The only criteria I have given him for a ring is not to spend much. Like, not more than "a few hundred". We both have debt, and I just can't justify spending a lot on a ring. I'm against diamonds -- but ONLY because I hate how expensive they are and the whole diamond market, not because I don't like the way they look! However, heirloom diamond ring given for free? Sure! That's the best kind of engagement ring, imo! My concern is that he misunderstood this to mean I don't like diamonds themselves... we never really talked about it, but I did comment on articles about the diamond market and have discussions with mutual friends that he might have seen.
So bottom line, what is upsetting me is that he spent a lot of money on something that I think was very wasteful, and unnecessary. There is also a petty annoyance nagging at the back of my head that darnit, I really liked the diamond version a lot better!
The other reason this bothers me, is that if we get married, we join our finances, and so the money he is spending right now on this stuff, rather than paying down debt, will affect us both later as well. We have talked a lot about money, and we are really both on the same page with money and have very similar values. We are both pretty conservative when it comes to money, which is why this is also so upsetting.
I don't know what to do. This has actually made me kind of angry. I'm a terrible liar. I won't be able to hide from him my disappointment in his choice to spend so much money on something like this, and I'm troubled by his decision to do so. When he asks, how on earth do I tell him that I'm disappointed and hurt that he would waste so much of our money just to swap out a diamond? My only criteria was that I wanted the ring to be inexpensive. 
He had the least expensive it gets, free, and then dumped a bunch of money into it to make it less attractive. Uhg. Just, so frustrating.
I hope someone has some magic bit of perspective for me that will make all these nasty feelings go away. And before anyone scolds me, don't worry, I DO plan on confessing at the appropriate time that I saw his email.