Do not use natural stone in a steamer. It can be done, yes. I have seen,
and repaired wineries with natural stone steamers. After a few years the fill
is popping out, the surface is etched. A lot of this has to do with the water
in your area. The chemicals added to it and the content of calcium.
Keep in mind this scenario is a continually running steamer for the most part.
Damage in this case is the easiest to find. Most residential steamers do not
see the same kind of abuse.
As far as construction goes, you may want to check up on kerdi. It is a vapor
proof barrier applied to the surface of your substrate. In fact I used it on my
steamer project and I typically install old school mud work. Here is a link
to the thread here with pics of my steamer project:http://www.johnbridge.com/vbulletin/...=kerdi+steamer
If you decide to go the Kerdi route, do read through
my thread there are lots of good pics and info from myself and other people.
Best of luck.