Any resolution on what's causing the dark lines, or how to address them? I am having a similar issue on a shower wall tile re-grout. Most of the white non-sanded grout came out fine, but in a few spots it appears that "something from below" leached into the white grout causing it to become grey and dark in spots.... and it occured quickly within the first 3- 6 hours that the grout was drying. In a few spots, it became rust color, as if the red ceramic base of the glazed tile was bleeding into the grout. Any thoughts?

That would not be a tile that I would ever use in a wet area. There are too many places for soap and skin cell residue to cling. That is difficult enough to clean off of a flat porcelain tile. Put the additional recesses in the mix, and combine it with marble, which has specialty pH balanced cleaning care requirements, and someone is going to be very unhappy in a short period of time. Either the soap scum will win, or someone will break down and try something unsuited to be used on marble to get rid of the scum.

Yes, there are DIY ways to improve the appearance of the grout and some of the etching. And I can understand you not wanting to deal with this bozo. Only you can decide the point at which conflict avoidance tips the scales on your personal cost/benefit ratio. That said, ask yourself.... if this is what I can see, and the tiler is defending it, wth is underneath it all that I can't see? This could be way more than a cosmetic fix. Aside from the sloppiness of the grout, I'd also be concerned with the grout at the ceiling line. I presume he also grouted the shower floor joints at the wall, too. Hard to tell what the vertical joint is from the pic. The point is, wherever there's a change of plane, it should be caulked. None of this suggests your tiler would have been any more conscientious with water barriers, shower pans, etc. that are now tiled in. Before you touch anything, be very sure you're preserving your rights along the way. In many locales, you have to give the contractor a chance to fix things before making any claims against his insurance, initiating legal action, withholding money, or filing a complaint with a licensing body.

The photo I posted above is a bonding flange drain that was shown in one of Mr. Starykov’s articles on this subject. When I saw the orange membrane, I assumed it was a Kerdi drain. My post should just refer to bonding flange drains in general. Sorry for the confusion. I am a newbie re: shower pan construction. I‘ve just been doing a lot of research recently in order to avoid having my marble discolor due to water retention. I have read several discussion threads where specifically Kerdi drains + issue with water pooling around the drain have been discussed. These included comments by well respected professionals who would be able to identify a Kerdi drain. There is always is the possibility (probability??) that the dams around the Kerdi drains were caused by installation error. Anyone who spends even a little bit of time on Houzz looking at bathroom renovations knows all too well how often these shower pans are not built correctly! I think catbuilder has “nailed it“ in the above post - there may be (i) too much thinset used to set the drain, (ii) a dam that’s been created around the drain (drain wasn’t firmly set into the the thinset?), or (iii) a valley in the deck mud around the drain. Catbuilder clearly has quite a bit of knowledge re: shower construction/proper waterproofing methods. I do have a question for you - is the marble tile that you’re having an issue with the same as the marble installed previously without issues? There have been a lot of discussions about how the particular quality of the marble tile being installed may have some bearing on the rise in marble discoloration issues. There is a lot of marble being sold in the marketplace that is labeled as ”Carrara” but is actually not from Italy. My understanding is that the density/porosity of marble can vary quite a bit between different varieties. It also can vary somewhat in the same variety. If I hold up two of the Italian Carrara tiles I’ve recently purchased for my shower walls to a light source, certain tiles are much more translucent than others even though they were sourced from the same area. This would contribute to the amount of water being retained inside the actual stone. Then again, if you have a dam or birdbath around your drain, your marble tile is going to soak up some water regardless of type! I REALLY want a marble hexagon shower floor - but all of this is making me VERY ANXIOUS. I specifically decided not to purchase the standard Carrara hexagon mosaic tile that I really wanted since white marble tends to have more issues with this - but if my shower pan has any type of water retention issues, I still will wind up with discolored tile.