How can I tell if my countertop is quartz or granite?

Look at the pattern across the whole run rather than at one spot. Granite has veins and mineral flecks that change direction and density, and the pattern never repeats. Quartz has a more even scatter, and on a long run you can often spot the same cluster twice. Check the edge of the sink cutout: granite shows crystal structure, quartz looks uniform all the way through. A water test is the other tell. Leave a puddle on an unsealed granite for thirty minutes and it darkens the stone; on quartz it sits there and wipes off.