My husband is a metal worker and he told me it was carbon steel. To be honest , I don't the the difference between carbon and stainless steel, but I can ask him and get back with you. I have had it several years and it has never turned colors or anything, but I don't wear it. I prefer rings with bling.
My husband says this is a high carbon stainless steel. The steel is a 410 grade stainless that will attract a magnet. He said it's highly unlikely to rust. I have owned it several years and it has never rusted on me.