If you trust your tile guy go with what he says-if he is competent he should know and be able to explain the difference. Sanded grout is best for bigger joints-(1/8 inch or more) there is less shrinkage than with unsanded grout. I like a smaller joint with the subway tile so I would go with the unsanded for that-especially if your tile is shiny then you don't risk scratching. Also it is very common to add the sealer in the grout and easier. If your mosaic tile is stone you should seal it also- it is not hard -I do it at least once a year on my honed limestone counters and it works surprisingly well.

Polished marble mosaics need unsanded grout; sanded grout could mark the surface. Stone sealer is usually applied over the surface, he will seal both the marble and the grout.

Yes unsanded grout!

Unsanded to protect the marble and the smallest grout lines possible, BTW not the best choice for shower walls.

Laticrete Permacolor Select is what we would use and you don't have to seal it. "An ultra premium, factory prepared, polymer fortified unsanded Portland cement base grout designed for highly-delicate surfaces, such as high polish marble ..."

@PatriciaColwellConsulting can I ask in your professional experience about how many people percentage wise run into problems with shower walls that have polished marble? I've read from many people who say they have had it for years and years with no issue, then I read the occasional bad story. I just love the look for my master bath and have read polished on the wall isn't a problem if it's sealed properly because polished doesn't really absorb as much water, but I don't want to be foolish.

@DragonflyTileandStone do you seal the tile before and after your grout to reduce risk of grout mixing into the marble?

@jrst what do you clean your marble with? Have you had much trouble with etching?

Seal before so that you can easily clean the grout from the marble, and then yes, after as well. Marble has been used in baths for centuries. Keep in mind that there are various grades of marble and it can make a difference on several levels. All the hoopala about marble staining is not a concern for most of our clients. But then they use the correct cleaning products and avoid harsh products like hair dyes, etc. One client had hard water and after about 10 years, before they put the house on the market, they had a marble restoration company come in and buff and polish the walls (about $400). New again. Most clients have no issues at all. Use PH neutral cleaners. We like MB Stone Care (MB-5), but there are many. More important for ANY shower surface is ventilation (a fan that is actually USED), and a quick squeegee and wipe down with a microfiber towel after use. It only takes a minute and is the single most important thing you can do to maintain your shower surfaces.

Tile spacing dictates whether or not sanded vs unsanded grout is used. If your marble is high quality, with 1/8” joints, then unsanded can be used. If the joints are wider, or dictated to be wider by the poor quality material often seen on mosaics, then sanded should be used. But microsanded. Small, smooth round sand that doesn’t scratch, and lends body to the grout so it does not sag and shrink out of the joint. Undanded has that as an issue when used with wider than 1:8” joints.

Mapei Ultracolor Plus FA. Use it for everything stone.

@Dragonfly thanks for the helpful advice. Much appreciated!!

my marble doesn't have a shiny finish-not sure how well you can see it- I was told to use simple green to clean it and I seal it with the regular stone sealer I got at HomeDepot. The feature strip is about 12 inches wide so its not a lot to take care of. The surrounding tile is white shiny subway and it is virtually maintenance free. My tile guy talked me out of the marble subway tile. It would have been beautiful but this is a beach condo and I wanted easy care. I was afraid the water would stain it a little yellow and didn't want that. No etching and the tile guy used unsanded grout for all

Oh that looks very pretty. I can definitely see if it is a second residence you wouldn't want to add more maintenance. I have the bevelled subway tile sitting in my garage because I've gone back and forth on it, but I really want the marble and ordered it. Are you saying that hard water can turn the actual marble yellow? I have heard of it turning white grout yellow, but not the stone, so now I'm a bit more nervous. I wish some of this was easier...lol

spend some time with a knowledgeable stone person and see what they say. I have to say that when it comes to making decisions about form over function I frequently go with what I think will be beautiful!! If you know what to expect ahead of time that helps-good luck