The “water ring” that you have on your stone  is a marks of corrosion (etching)  that could be made by various acidic or other pH active liquids that became in contact with the surface of the stone, i.e.: drinks, vinegar, salad dressing, lemon juice, tomato sauce, orange juice, some generic cleaning product and so on through a long list. There's no product on the marketplace – sealer or what-have-you – that could prevent such surface-damages from happening. You will need the services of a bona fide stone restoration contractor, who will have to hone and re-finish your stone. You could also take a look at MB-11. Keep in mind, however, that MB-11 is the only product in the MB line that does not come with a 100% satisfaction guarantee. This is due to the fact that if the etching is too extensive and/or too severe, the whole tabletop or floor, or whatever will need to be professionally honed before polishing. It will help a great deal though, and if some handy person in your household can use a car buffer, it would increase the performance of the product dramatically. There are also other marble polishing products available on the marketplace, but they all have the same limitation. May I ask you now to please read and e-sign our Statement of Purpose at: http://www.marblecleaning.org/purpose.htm?   :-)

Ciao and good luck,

Maurizio Bertoli