Bookmatched stone slabs can appear to be seamless with a good fabricator. But that size will dictate a seam in materials.

The trick is to get it into the space and installed since it will be a bitch to get to the ceiling. Get a quartz company to come out and see what they think. Porcelanosa has some amazing huge format tile that I think would look cool.

Slabs in marble or quartz, and I would stick to the REAL thing. A good stone shop, and a fabricator. That's how you get it. there is no other way to get no grout, other than to not NEED grout.

Patrica and Jan, you seem to be ignoring the OP's first criteria, which was "seamless". No quartz, no stone, no tile, will come remotely close to the seamlessness of solid surface. There are some amazing stone look-alikes and in a vertical application such as this, a gloss finish will look and perform flawlessly.

What I didn't notice was the 7 x 9 foot dimension. So, I will be buying my plane ticket to Dubai, so I can watch them bring it into the palace : ) I think that much "seamless" is actually overkill. I'd INTENTIONALLY break marble with ( three ) well designed seam placement in this instance.