QUESTIONWould you recommend installing polished carrara marble into a steam shower? If not, what natural stones would you recommend?
ANSWERANSWER - I have seen Carrara Marble installed in showers and steam shower successfully, but I have also seen where there were staining problems.
Many marbles contain pyrite (iron sulfide) minerals which is a natural condition. If those marbles are exposed to excessive moisture it can cause a rusting or orange/yellow-like stain. Generally speaking if caught soon enough it can be cleaned off with proper cleaners.
Any marble that is adjacent to or in contact with a metal, the metal will rust causing the marble to rust in those spots. Mechanically attached marbles should use stainless steel clips that don't rust or other stainless steel attachments. If the marble has embedded metal rods for support they should be stainless steel or fully encapsulated in an epoxy resin to prevent rusting and staining. Metal lath in a mortar installation can rust; even if it is galvanized, so it too should be stainless steel.
The key is to install the marble in a manner so it isn't exposed to metal or excessive moisture. Obviously a steam shower has substantial moisture. If the marble is installed directly over a waterproof membrane that can minimize the moisture exposure and if there is adequate drainage will help too.
You can't guarantee that some sort of staining can't occur on a marble when installed in a wet environment. Assuming you have installed the marble correctly, you can have a proper maintenance plan to minimize the potential for staining. Sealing the stone on a regular basis so water always beads off of the marble surface helps. Toweling down the shower after use will help. If you start to see some yellowing then have a stone restoration company come in to do a deep cleaning and refinishing if necessary.