The most common oxide on the Earth's surface, constituting 12% of the crust by volume. Quartz is a crystalline form of silicon dioxide (SiO₂). When cooling magmas solidify into igneous rocks and when preexisting rocks metamorphose into new mineral assemblages, the crystallization of quartz is favored by the sheer abundance of silica in the crustal environment. Among the igneous rocks, quartz is especially common within granites, granodiorites, pegmatites, and rhyolites. In addition, quartz can be observed in low- to high-grade metamorphic rocks, including phyllites, quartzites, schists, granulites, and eclogites. Because hydrothermal fluids are enriched in dissolved silica, the passage of fluids through rock fractures results in the emplacement of quartz veins.