That is undetermined. I've had them for years in my jewelry box and they have not tarnished or turned slight green like my other costume jewelry. They've stayed bright shiny silver. But, they have no markings on them like sterling would.
Actually now that you mention it, they would be perfect for a guy/girl set. The longer thicker one looks more like a guy's chain which is why I never wore it. They were part of a set of 6 that my husband bought me.