It's a complete crapshoot. The iron won't show up until it's been wet a few times. No natural stone is recommended for a steam shower. It's porosity in a 100% humidity saturated environment can grow mold and mildew and other icky things. Sealing won't do diddly. It's to keep liquid stains from penetrating. Won't keep out steam vapor. Or mold spores.

Hollysprings - I appreciate your feedback! What about all the people on this forum who have put marble in steam showers, then? I have heard of them complaining about iron spots. But never mold spores.

More excellent advice from hollysprings.

Holly - I have three windows in this shower and live in So Cal. The shower will be well ventilated. According to my contractor, Dry Treat sealant was developed in Australia to protect underwater wood piers and for 15 years. My contractor said to seal the marble with this and to seal the wall behind like a monolith. He puts in marble steam showers all the time.

Zagyzebra- Ask that contractor for names and addresses of people for whom he has installed all these showers. Find some that are a few years old and ask to go see them, or at least call them to get their "real" opinions, not just what they said a couple weeks after the install before any problems might show up.

-Babka

Hi Zagy

I had Emparador dark and light marble on every surface in our master bath for nine years (regular 4x6 walk in shower, not steam - but used daily by at least two people). We never had an issue with rust. We had a flood that required a gut to the studs. I seriously considered going back with all marble again but wanted to lighten it up using a white marble. While doing my research I read to order samples of the marble under consideration and soak for three days immersed in a bucket of water. Then, let dry for a few days. Rust spots are supposed to appear if the marble has a high iron content. We did just that with all manner of white marbles and the crystalline looking (inexpensive) marble imported from China had orange streaks and spots. The Calacatta and Carrara from Italy as well as Greece origin white marble tiles did not. I imagine it depends upon what part of the mountain and area of the world your marble is coming from. In the end, I didn't want to chance it or deal with etching or staining and used a gorgeous porcelain Calacatta look-alike in the shower and real Itallian Calacatta slab on the counters. The tile guy isn't finished yet - hope to have photos in two weeks or so. Good luck!

Great information Jacqueline5. What was your experience with the chinese crystalline type marble after you soaked it?

Jaqueline -- Yes, I began to get the same information you gave. I think this will be very useful for people considering a marble shower. If I get any more tips, I'll be sure to return to this forum to share.