Crl, our granite shower base is a breeze to keep clean. I have heard some say that black stone shows water spots but that may be for shiny black stones not honed. We have had our granite shower base for about 6 years.
Just I case you are interested here is a cross section of how our shower basin was fabricated. It may not look like it from the cross section schematic below but it is single piece of stone. The holes for the drains were precut at the fabricators shop before it was brought to the house for installation
Good luck with your project.
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large field tile
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