If you use glass, it has to be tempered glass. You'd really have to hit it to break it.

I agree with you about using a porcelain tile, and as for marble, you can pretty much figure whatever soaps and shampoos you use will take the polish off them, but I doubt very much you'll notice it. If you use granite tile, though, that polish will last a whole lot longer.

As far as with respect to the finish in the rest of the shower, or even the rest of the niche, that's immaterial. The shelf is there for a utilitarian purpose, as opposed to aesthetics. For the most part, you'll have things covering the shelves anyway, so even the most obsessive compulsive person wouldn't really notice it.

Our niche has 2 glass shelves. The glass is 4 inches deep and 24 1/2 long.. 1/4 inch thick and tempered. The tempering part is so that if it does happen to break, it will break like a car windshield; hundreds of pebble sized pieces, no shards.

When I went to my local glass store to place the order, the owner was there.. 3rd generation glass and mirror guy.. he said the only support we would need for the 24 1/2 length would be at the ends. The tile ends rest on a tile.. the shelf blends right in with the surrounding grout line.

We chose glass because we set a waterproof LED light in the top of the niche, it shines down through everything and looks pretty cool at 3am when you're washing off the day's dirtiness.

In our master bathroom, we asked Bill to install white Thassos marble shelves for us that he cut from 12 x 12 marble tiles. It's beautiful, but it was really expensive (although it would have been a lot less had I chosen a different marble!)

In my daughter's bathroom, I am on a very, very strict budget so I need a cheaper option. I have an idea I hope will work: I hope to use the prefab marble threshold pieces you can buy at Home Depot, etc. They're about 12" long and less than $5 each if I remember correctly.