You can use ready acrylic shower bases, or may be a marble slab, 1 piece so no worries of cleaning grout.

my choice will be Honey Onyx 12"x12".

What is the upkeep for marble? Does it have to be sealed?

One piece of marble is going to be way too slippery. Have you looked at the river rock flooring? It's incredibly comfortable to stand on when wet. Otherwise, a one-inch glass tile is always pretty. Just spray Tilex every once in a while and all is well.

Hard water may stain any natural product. You might want to check that out. There are a number of Porcelain tiles on the market that look very much like different stone products. I used one that looks like Limestone and one that looks like Marble. They come in different shapes and sizes for a more contemporary or more traditional look. Both of mine have smaller tiles for the shower floor for safety
Look at a good tile store. Good luck .

Kobley has the right idea. Porcelain tile will give you the durabbility and maintenance features you need with hard water. The choices are unlimited. Natural looking porcelain has dimension and depth since the color is in the body of the tile. Slate, granite, and wood grain designs in lots of shapes and sizes can be combined and you can achieve random natural patterns, or detailed formal designs. It also plays well with glass and metallic tiles.

Keeping it clean is a maintenance issue, all materials have their ups and downs. Rectified or gauged tile is cut to be close set, minimizing grout lines....the thing in tile floors that collects the dirt and shows it off. A gray grout minimizes the show (compared to say, white) but needs to work with the tile. Mosaic tiles set on mats are one way for tile setters to save labor over hand set mosiacs. But all showers, except the newer trench drain styles need to slope in four directions for drainage...from the wall to the center drain...and mosaics and smaller tiles are the way to get that done. Grout and smaller tiles also improve slickness of the floor, making falls less likely. Trauma from falls in baths and showers can be deadly, especially for the elder population, so the addition of grab bars augmenting the texture of the floor can be helpful. We often set blocking for future grab bars when we rough in framing for a shower. Waterproofing is a given, as is sealer, regardless of the tile material.

Thanks for the informative responses. So many details to be considered when building.