I was never aware that after installing natural stone tile that it needed to be sealed. My tile guy said that it had sealer was included in with the grout installation. After about three months, it looked very warn and the grout showed darkness in the grout lines, only where the grout had any traffic on it. It was like was still dirt getting in the grout and in the tile itself. I was totally dissatisfied, then I researched and found out that the floor needed to "cure" for at least three days befor sealer could be applied, so I know the sealer he applied wasn't enough. I hired Mike at Mass Service Solutions and he came in cleaned my grout back to it's original color "white, unbeleivable!!!, after almost a year, and he sealed it ". now it has a very nice sheen, not that chalky look it used to have. I would definitely recommend him to anyone with this same issue." Now I can finally have a floor that not only looks clean but is positively clean down to the grout.