Would a pizza stone assembled from multiple pieces be smart/useful? I have access to an old marble quarry, and I would like to go cut a stone there. It seems too complicated to cut a huge piece (even more considering I need to sand it and all) so I wondered if I could instead cut "bricks" that would be easier to handle (and even to store). For example, 20 centimeter pieces I could assemble depending on the size of the pizza and when I'm done put it away. Has someone ever attempted this? Or is there some fundamental rule that would ruin my plans? I expect to have something less performant, but not too much 
America's Test Kitchen did tests of pizza stones, and one was actually a set of bricks.  They found no problems from the seams ... which makes sense, considering that brick ovens would've been made from bricks, not large slabs of stone. What I'd be concerned with is that you're talking about marble.  It's not the most dense of stones, which means it won't hold heat as well as other stones, and it will absorb liquids.  It also has veining, which are basically fault lines running through it.  If you accidentally heat up a wet slab, you risk it cracking (possibly explosively, if you heat it up too quickly). As such, you would want to heat it up to just below the boiling point of water, wait for it to dry out, and then crank the heat up on it. 