This Item is used with a magma drop to create Glazed Magma, this in turn is used with other items and finally a master potter 250 can create a Ceramic Incence Burner of Ro that has a focus effect of Improved damage IV, a nice item but takes much to get the parts to create. I would like one for sure :)
It is stackable, and, if I remember correctly, NPC merchants sell it for about 13pp.
Of course, then you have to find someone who can use it (I have 5 sitting on my bazaar mule at 2k each -- have no idea about pricing, but no one even asks about it).
If you saw it for sell at a merchant it's because someone sold theirs to it. The only place you can get the granite is in Pofire. the fire elementals and doomfires drop it.