From Laticrete's website: Certain types of stone (e.g. Thasos White marble, White Carrara marb/e, other statuary marbles, and other absorptive stone) may draw the grout liquids into them causing a "picture framing" effect which will darken or discolor the stone. Coduct a test area to determine suitability and acceptability. Here is a link that might be useful: LATICRETE

I'm not a tile expert but I did a lot of research before installing slate tile floors. What I read was that you want to avoid sealing before installing the stone tile because you want to make certain the stone tiles bond properly with the thinset on installation. When you seal before grouting, you want to seal the top of the tiles to avoid getting the grout stuck on the top (makes cleanup easier and decreases possible haze issues if you don't clean up quickly enough). But you do not want to saturate the grout joints because you want the grout to bond properly. So I would recommend installing, then sealing, then grouting, then cleaning, then sealing the grout. Sealing is quite easy.

Yes the grout should be sealed. Helps protect the grout. Helps delay the inevitable. As far as I'm concerned, sealing grout is one of the biggest scams in the industry. I do have to qualify that statement, though, by saying that I'm in the minority in that thought. The thing is if you decide to use a white or off white grout-- something real light-- in high traffic areas, you're still going to show traffic patterns. You're still going to end up with staining, and the grout will still slightly darken as it gets older, from minerals and so forth in the water as you wash the floor, not to mention the dirt that gets rubbed in as you clean. There's just no way around it. You're much better off with a medium to darker color grout that won't show it as much. In the long run, you'll be alot happier. Although I've been working full time for 29 years now, I grew up in the trade, setting my first tile when I was 10 (41 years ago), when my father built the house I grew up in. but my father (and grandfather, for that matter) was a tile contractor, as well, and I've never once-- not in my Dad's house, or my own, nor in any of my 4 sisters' or my brother's homes, have I (or they)ever sealed one inch of grout joints. As for sealing tile, other than natural stone, as far as I'm concerned, the only tile that should be sealed is crackle glazed tile. There are some old generation porcelains that needed to be sealed because they did tend to stain. But the newer porcelains, if you try and seal them, all you'll end up with is tile with a heavy white haze on it that's tough as nails to remove.

@dostk did you ever resolve this? We had a white and gray honed marble mosaic installed and after grouting, the whole area looks completely different. The stone doesn't look anything like the uninstalled mosaic, now it has almost a dull/frosted glass (not shiny) look to it ... is the best way I can try to describe it. Basically it doesn't even read as marble to me anymore. As far as I know, our contractor's installer did not seal it before hand (it was still in the boxes and packing sleeves) and used a sanded grout. The option of sanded or unsanded was never mentioned to us and as newbies we didn't know to ask. Is there anything that can be done to bring the original look back or is this just expected and we should have done a mock up as someone mentioned before?