It's a bit after the fact, but here are my thoughts on natural stones.
If you go far enough back in time, you'll see posts of mine professing the superior attributes of soapstone. I don't regret recommending it, because, at that time, there wasn't anything better available, but we have far better options now. Mother nature has no quality control. Anytime you walk into a stone distributor, the stone you purchase can contain trace elements of just about anything AND it can have defects that aren't visible to the naked eye. Natural stones are always going to be a gamble. You can hedge your bet a bit by going with soapstone, and identifying it carefully, but there's still always a chance that it will either have flaws that cause it to fail down the line or be comprised of materials that don't give you the thermal conductivity you're looking for.
Granite, for cooking, isn't just a gamble, it's putting your life on the line. Granite has incredibly poor resistance to thermal shock. It's like baking with glass- and not Pyrex either. It's not a matter of whether or not it will fail, but when it will fail and how violently. There are certain materials that have no place being near heat and granite is at the top of the list.
The idea that you can walk into a quarry, any quarry, and find a stone to bake on needs to die, once and for all. These stones served our ancestors honorably and they still have beautiful non heat uses, but, in this day and age, anything that goes into an oven needs to be engineered to do so.
Steel is really your best bet, IF you have a top heat source. If this is a bottom heat only scenario, then the conductivity of the steel will cause the bottom of the pizza to burn long before the top is done. In that setting, you want to handicap the bottom heat transfer with a poor conductor- like quarry tiles.