For a while, at least:
When the rocks cracked into small pieces from repeated heatings, they were tossed out and the oven pits re-lined with new rocks.
Granite is a composite stone, made up of lots of other types of rocks, as opposed to slate, which is a single rock, made, as it happens, by heat. I would be concerned about slate's tendency to break into layers, though.
I got my pizza stones for free when a wave of Pampered Chef parties invaded my neighborhood years ago
I had one three years in a row myself. Then I decided I had enough kitchen gadgets, at least for the time being.