Husband's wedding band?

Can anyone provide some input as to what this line is on my husband's wedding band? He just cleaned it (with toothpaste??) and the line is still there. Btw his wedding band is sterling silver. He's only had it for about 8 months and has worn it in the pool..? It's not a crack as far as we can tell. When we run our nail along the inside you can't feel any roughness if t were a crack. Thank you so much!