I have a silver ring as my engagement ring, and it's also going to be my wedding ring. I don't need anything fancy. If I want a fancy ring one day I will buy it myself for decorative purposes (it won't be a diamond either). The whole idea that diamond rings need to be expensive is a marketing ploy, and diamonds are a terrible industry.
The idea that someone should spend thousands of dollars on a piece of jewellery not because they really love it or can afford it, but because it's socially accepted - that's so sad.
I like having a ring to symbolise my commitment, but diamond? Unnecessary.