Quartz is the crystalline form of SiO₂ that is prevalent at the earth's surface. In granites, quartz can be clear, or white, or yellowish, or light gray, or in some special instances, light blue (generally only 1 color per granite). The granite image you were looking at was picked for use because the quartz is somewhat gray, which makes it easy to tell from the plagioclase. When looking at real granite, quartz typically has a slightly greasy luster that makes it easier to identify.

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