Have you done a search on the forum? Lots of threads detailing all the reasons why you should not use marble in a shower. Not a stone expert at all, but it looks like your marble has iron in it which is rusting with the exposure to moisture ... it will only get worse.

Mongo, I don't believe the discolored spec is related. Pretty sure the two pieces were most likely cut from the same tile, as all those on the curb were split tiles.

This condition doesn't appear anywhere else. I'm assuming the repair is to remove these two tiles and replace them. How difficult is it to dig the tiles out from a mortar bed?

I'm in agreement with Mongo: you have an exposed fastener in that location: it's rusting and leaching outward through those two tiles and the grout. No amount of scrubbing will make this go away, and it will continue to spread. You've correctly surmised that the only permanent fix is to remove those tiles and remove the fastener. Do you have any pieces left over from the job that can be used as replacements? Unfortunately, you won't be able to salvage the damaged ones. Fortunately, a competent tile guy should be able to take care of this without much difficulty.

Rusting fastener has my vote. Although there should not be any fasteners there as they would penetrate the waterproofing.

Thanks for your responses. Think we will have the tiles removed and replaced. Hoping we don't have to take the glass out to do so.

It looks like you should be able to get this fixed without having to remove the glass. I would think that the worst case would involve the two pieces of stained tile and (maybe) the one above them on the top of the curb. Do you have replacement pieces?

Think I've got one piece left...maybe two. I can order more if I have to. Thanks.

It could have been in lots of worse places! Good luck.

Let us know how it turns out.

4th vote for exposed oxidizing fastener.

Update - We finally had the repair done. When the tiles were pulled off, the tile guy found a broken piece of wood about 6" long. I recall that the original installer had used full length boards as temporary overnight supports. We speculated that when he removed the board, part of it broke off and he either wasn't aware or didn't bother to retrieve it. In any event, the wood was quite wet and had obviously been leaching through the tile for some time.

Problem solved! Thanks so much for all the input. Nobody got the right answer, unfortunately, but at least we got it figured out.

But how was the board gettig wet? Are you saying there was a piece of wood on top of the waterproofing and under the tiles? What kind of tile installer would allow that?

Sorry if I wasn't clear. The board was placed at the base of the curb, on top of the mortar bed, to temporarily support the tiles. The next day he removed the board, although in this case a chunk of the wood broke off.

Why he used wood instead of a metal bar or something else is beyond me.

We typically use wood strips as ledgers to make the bottom row of tiles level and support the upper rows. The ledger is removed, the waterproofing restored, and the lower tiles are installed.

Thanks for clarifying. Hope that's the last surprise for you. At l least it was in an easy location to make a repair.