It sounds like some of your friends have the problem, not you. You show no signs of having "low self-esteem"... You just aren't materialistic like they are. I understand exactly what you mean. It doesn't matter what the ring looks like or how much it costs -- it's how much you love the person who gave it to you and how much he loves you!
You can be wearing a rock on your finger so big that you need help lifting your hand, but if your guy doesn't love you it's not worth a dime!