Unfortunately, sealing doesn't help, and it is only the white marbles that have issues. Other marbles are fine for showers as long as you know how to maintain it. Marbles, limestones and travertines are all calcium based products and most cleaners from grocery and big box stores are acid based which will eat the material, etch and take away the polish. With all natural stone, as long as you seal it well (make sure you use quality sealers -- Aqua Mix products or Tech's Bulletproof sealer-- both rated top sealers and last 10-15 years) and squeegee it every time you shower, it will be beautiful for a very long time. If you don't want to maintain it, the technology that they have in porcelains anymore make it very hard to tell the difference :)