Not all engagement rings are created equal. Despite what you see in Katherine Heigl films.
No, these days there's no rule that says you have to have diamonds, no rule that says a platinum band is better than a gold one. Basically, like everything else, the modern engagement ring is a "you do you" situation.
Except someone else is choosing it for you.
Annoying that, isn't it?
Nevertheless, just because you're not allowed to choose the final ring yourself (and tbh, I've got friends who absolutely have chosen their own ring because, well, they don't fancy the prospect of pretending to like a ring they've got to wear for the rest of their life) doesn't mean you can't drop hints.