You could use Vermont White quartzite which is harder than marble and granite.

I would stay with white. Quartz would give you a plain white with no veining or speckles to compete with the marble.

Consider a slab of clear or frosted glass with white undermount sinks. Calacatta typically has a greenish-grey veining as well as a grey. The tint of the glass will blend nicely with your tile, eliminate the need to coordinate another white material, and tie in well with your glass shower enclosure.

Have you browsed the bathroom photos for ideas? Type Calacatta in the search box to narrow down the photos.

Here is the Vermont White. I couldn't get to the picture yesterday when I posted.

Another material to consider is milk glass...not crystallized artificial stone that is called milk glass...but white colored glass. It comes in slabs 3/4" and 1 1/4" thick. It has a depth of coloration and reflectivity that works with the porcelain whites in the "faux marble" look and does not compete with it.