the whole theory behind "a stone" is to preheat it, plop the pizza/bread/whatever on the hot "stone" - the stone "holds" heat - which it "delivers" to the bottom of "the baking item" making in the case of pizza, a crisper bottom to the crust.
so the theory hold that virtually anything that "holds heat" well be it marble, granite, slate, steel, floor tiles, depleted uranium..... would fulfill the purpose.
the "glitch" comes into the picture as follows: natural stones often have 'fault lines' in them and their composition can vary from inch to inch. what this means is: they may crack due to uneven thermal expansion when preheating and/or plopping a cold mass on them (uneven thermal contraction)
stuff like a 1/2 inch steel plate ain't no way gonna 'crack' at home oven temperatures. but, it is a tad on the heavy side....
"man made refractory materials" - that's the typical "pizza stone" sold - they are homogenous, do not tend to crack break on rapid temperature changes, and badda bing badda boom, are lightweight.
as for floor tiles / terra cotta 'in general' there's the issue of 'is it food contact safe' ref heavy metals, etc.
natural granite usually contains some (honestly, very minor/negligible/ignorable except to 'freak out types') degree of radioactive materials.
marble & slate less inclined to contain such.
all that aside, all these kinds of materials will work - some may have a "longer life" than others - my round ceramic pizza stone is at 20 yrs and counting.
if it's cheap, and doesn't break, you got a winner.