Stick with your guns. Why would you buy a diamond when you're so opposed to it for perfectly valid (although potentially subjective) reasons? Does the perception of other people that you are "too poor to afford a real ring" matter to you so much?

My wife was appalled when I told her I refused to buy her a diamond engagement ring. "Why wouldn't you want to give me a diamond ring? I couldn't marry someone who wouldn't give me a diamond ring."

We're married now and she doesn't even wear the sapphire ring that I bought for her because of chores, etc. Sure, she had to explain to almost everyone why it wasn't a diamond ring but that just means that she had one more topic to go through with her friends.