Porcelain, high density, at least commercial grade 4, smooth surface. After months of research, I chose a Crossville tile that has been on the market for a decade. Large tiles for the walls, and a mosaic for the floors -- it is hard to find a porcelain mosaic because porcelain is harder to cut than stone.

Part of my research was investigating anti-microbial tile, made by a couple manufacturers here and more overseas. After talking with the chemist at the standards institute for porcelain tile, I concluded that this new technology was very useful for exterior applications, but that in a shower, the risks of absorbing these materials into the body (with moisture and heat) were not worth the marginal benefits, especially in a residential application. In an NFL locker room -- I might specify the additional benefits of anti-microbial.

Porcelain tile does not look like stone, but a porcelain walled steam shower has the added benefit of a requiring a less powerful steam generator. Stone is next and glass takes the most energy. So with porcelain you save twice -- first in the less powerful generator, and over time with using less energy. So in a steam shower, porcelain is more green than the natural stone.

Porcelain is also easier to install, unless you want the large size tiles -- anything 12x12 or smaller is easier than larger in a wall application.

Also, to keep it clean, consider the anti-microbial grouts -- and give your family a squeegee! Just a few seconds to remove most of the hard water has made a huge difference in the amount of chemicals and elbow grease it takes to keep our shower clean. Hand towels work too -- a few swipes, and a fan that has an automatic turn off can work, to keep the moisture out and the mold from growing. At least where I live now there is only one kind of mold -- in Seattle we had three.

Good luck -- I really wanted to use marble, or even granite, but my one stone shower that is so difficult to keep clean, reminds me to make it easier next time; there are so many attractive porcelain tiles -- you can find the look you want.

On the other hand my architect will renovated her bathroom with crema marfil and use bleach to keep it clean. I think she has a point. I use a light concentration of bleach on the mosaic marble floor of the stone shower once a month or so.

I would really like to hear from people who have installed "rock mosaics" on their shower floors or walls. I think my hard water would create water spots???? Ditto for slate -- is that easier to keep clean only if you do not have hard water? My friends in Seattle love their slate shower, but here with hard water I don't think it is practical.