1. Slippery, even at only 6" in size. Not small enough to have enough grout lines for slip resistance. 2. Soap scum and hard water deposits. All of that will show up almost instantly on a shiny black surface. 3. Additional lighting will be needed. A dark surface needs a lot more lumens to achieve the same well lit appearance. 4. It's not a traditional material to use for a shower. Probably would work great stylistically in a modern house. Not so much in a Georgian.

If you use the radiant floor heat as an alternative to a wall heater, the extra square footage is nice. Our climate isn't very cold but it is nice to have the chill removed by the floor when you step in the door to the room, not just the shower door. We have the whole floor done in radiant heat. Not a big deal to add the extra square footage. We have central heat but our house is a pier and beam foundation, built in 1935 so we still have some draftiness on cold windy days/nights. This floor heat provides a nice little addition in a room that tends to have a higher chill factor because of the tile if there is cold weather outside.

IF the 2 cm is available, I agree. Just that most times the vanity top is made from 3 cm stone, and usually they'll charge you for a second slab if you ask to have pieces taken out of a 2 cm slab to match. As for just mitering the tile in the niche, I did that around the other three sides in the plank shower I did above, but it's not my favorite thing to do, because it tends to leave a sharp, but weak edge that can be easily chipped. as for the shelf, There's a reason I went to using stone instead of tile for the bottom of the niche. That being that when the bottom of the niche is flush to the face of the wall, it tends to show soap drip marks down the face of the wall. With the stone shelves, I extend the shelf out about an inch past the face of the wall, to give what's called a "drip edge", so that any water or soap that falls off the shelf, falls to the floor, rather than leaving a streak down the face of the tile. And 3 cm is NOT two stones glued together. It's 1 1/4" solid stone. it's HALF again the thickness of the 2 cm stone.

I like the look of a simple backsplash using the countertop material rather than using a mosaic there. We used a 6" tall splash instead of 4". We just finished our master bath last fall and we used the Emser Lucente Lazzaro for one shower wall and the niches. Lazzaro has a lot of light greenish-gray-marble colors in it. Lucente comes in a lot of colors. My local showroom only had 2 colors on display, but the Emser website shows a lot more options. If you use Lucente I have one suggestion regarding installation. I don't know if this will occur with a different color, but with the Lazzaro color, there were some marble pieces that had black dots in them. That was fine as long as these dots occurred randomly, but it just so happened that some of them lined up in one area when being installed. The installer was so close to it that he didn't realize. His helper was standing further back and noticed. While things were still wet, he cut out the offending pieces with a utility knife and replaced them with a piece from another sheet. Just an installation detail I thought I'd mention. Our colors were: Emser Uptown Sugar Hill for field tile in 12 x 24" (it's a light gray/concrete color with some variation), Lucente Lazzaro linear mosaic for one wall and niches, QDI The Rock 3" x 3" floor tile (charcoal gray), marble countertop and tub surround with marble splashes, white vanity cabinet and tub skirt panels, a large linen cabinet stained in a medium brown tone and some floating shelves also stained, and existing wood floors. We thought a white and gray color plan might be too cold so we incorporated the greenish-gray mosaic and the stained cabinet, plus a teak bench in shower to bring in some warm elements.