We had a price listing some time ago. The prices of the granite sort of followed ingot prices *plus a 0* for difficulty to mine up granite as compared to ore.
Plain granite was something like 250 gp a rock.
DC granite was about the same or 300 gp a rock.
Shadow/Copper/Bronze were once netting 400- 450 gp a granite rock.
Gold granite was something like 500 gp a rock.
Aga 550
Verite 600-650 a granite rock.
Valorite granite, went like 700gp 750 to 800gp per valorite stone !
Anyhow I think the price list's been long gone thus not current, but I do remember it.
I look for granite often myself, even though I too dig it up, since some things take so much granite, I oft buy it all up too, when/if I see it, selling via of others to make more things with my carpenter.