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.

Would 4" square make any difference slipperiness-wise?

I was going to do a 2" carrara honed square floor in the shower to match the majority of what is in the rest of the bathroom on the floor, but after what we're going through with marble in the master shower floor I am rethinking it. We will have 12" carrara on the shower walls. Any suggestion for the shower floor?

I wouldn't put any polished tile on the shower floor. How about pebble tile in whites, grays and black?

What about using the polished AB as just a border in the shower to echo what you have on the floor, and then using something else (perhaps some matte white tile) on the rest of the floor?

I used AB for the curb (and vanity) but the shower flower is Daltile 'Continental Slate' (porcelain) in 3" squares. Here's a photo:

(Work-in-progress photo - not quite finished on left side)