925 Mexico Silver Ring with Dark Agate type stone

Beautiful 925 Silver marked ring with stone that looks like some type of agate Makers mark looks like TB-18 or TD-18. As far as I understand, the T indicates the city of the maker (Taxco), the second letter indicates the initial of the last name of the maker, and the number indicates the makers ordering in registering with the two letter combo (i.e., this person was 18th the register with the last name starting with D/B in Taxco) Size is approximately 7. Can take measurements of the ring on request.