growth ring

Hello, i recently noticed a black ring appearing around my tortoise shell I rescued him recently from my home of 15 years and have no idea what this is and I am confused as whether or not it is shell rot or if it is dangerous to him and what to do to treat it if it is please help me identify...

You can't tell her size in the pic, but TRUST me when I tell you this lady is getting big. She seems to have quite a few stuck scutes, she isn't pyramiding and is in good shape considering she was wild caught and bought at a show. Everything was even at 4.5 inches in size, but now with a LOT of...