Mine are TRAVERTINE corner shelves. VERY heavy feeling...they are being installed now. Very thick too. I got them at a local specialty tile store...

My tile installer made the shelves from the same tile used in the shower, for a perfect match. He cut the 12 inch tile on the diagonal, glued the two pieces together, bullnosed the long edge, made a dado and thinset them into place. Looks great. Wouldn't work with smaller tiles though.

We have Corian shelves in the tile, and I love them. Corian can be cut to almost any size. The tile installers found them really easy to install too.

If you're doing a tile (porcelain or stone) shower, I would recommend a nice pre-formed shower shelf (they also come in sizes to be seats) called Better Bench--we used that--it's a form that gets covered with the same material as the shower-- and it's nice and strong.

Here is a link that might be useful: they also make pre-formed recessed niches

Your local tile store should have a nice selection, either on the shelf or by special order. Since I used plain white ceramic ones, I was able to get them at Home Depot, dirt cheap. Eclectic-Ware.com carries Lenape's corner shelves, which come in a pretty good color range, and granite shelves too now. A glass store should be able to fabricate tempered-glass shelves for a lot cheaper than the Eclectic-Ware ones though, and provide you the mounting hardware as well. Any stone fabricator should be able to buzz up some shelves for you out of scraps; if you're having a granite countertop made, they should be able to use the piece from the sink cutout to make at least one and probably two shelves, depending on how big you wanted them.