There's no such thing as granite quartz. Quartz is a component of granite (the others being feldspar and mica), but granite does not contain a specific kind of quartz. The closest thing to that would be quartz diorite (which is basically just a diorite stone with a larger than typical amount of quartz in it), though it would have been used for building material, not decoration. Quartz is quartz... it's a crystalline mineral composed of primarily of silicon dioxide. It's the most abundant mineral on the planet. But all quartz is not the same. It comes in many varieties (mostly differentiated based on impurities in the crystal and crystal size). For example, the gemstone amethyst is a type of quartz, as are agates, onyx, prasiolite, jasper, adventurine, tiger's eye, carnelian, and citrine. You'll also fine rose quartz, milky quartz, and smoky quartz... in addition to the almost completely transparent and colorless pure quartz. You can buy it from just about any place that sells mineral specimens.
