I love my iron rings, I got them from a blacksmith. Rust is great, because if you let them sit, they punish you for it, if you use them very regularly, they never get rusty on the inside. From a more utilitarian perspective, the material was generally whatever was available from my understanding, so metal plays a larger part in my decision than what metal exactly, as long as it is not toxic like lead or DU

. Heavier is also not always better, but that is another topic, and I of course do not speak for any school other than my own.