I would avoid natural stone, especially limestone. Very porous, giving all those minerals a nice place to land and build up. I would choose a smooth-finish porcelain tile, and keep it clean with Murphy oil soap. Good luck!

I would avoid tile of ANY KIND all together, and go with fiberglass. Atleast them you can use CLR to clean the shower and keep it looking nice without worrying that you're going to ruin it.

I would suggest Swanstone. It can be lightly sanded to remove anything. I had that in a home I lived in new. Seven years later when I moved out folks couldn't believe the shower had ever been used. My present home has it in a 33 year old home and also looks spotless.

Daltile makes a super large rectangular porceline tile. They are 12X18. We couldn't afford solid surface in our master bath shower and it is an odd size, so that's what we went with. It only comes in white, but it has quarter rounds that match.

We had very, very hard water from our private well (400 ppm) and before we took on the two upcoming bathroom remodels we installed a water softener instead. It's made a world of difference in how easy everything is to clean (bathrooms, dishes, the kitchen, clothes, etc.) and makes our hair and skin feel great. Go check out justalurker's posts on the plumbing forum on how to size water softeners.