Quartz sand that is recently weathered from granite or gneiss quartz crystals will be angular. Amethyst is a violet variety of quartz. Common colored varieties include citrine, rose quartz, amethyst, smoky quartz, milky quartz, and others. Other linear polarizers exploit the birefringent properties of crystals such as quartz and calcite. Clocks keep time by mechanical resonance in a balance wheel, pendulum, or quartz crystal. Quartz crystal strain gauges are also used in geotechnical applications. Crystals are found throughout much of the Alpine region such as cinnabar, amethyst, and quartz. Applying alternating current to the quartz crystal will induce oscillations. An early use of this property of quartz crystals was in phonograph pickups. It is cryptocrystalline, consisting of fine intergrowths of the silica minerals quartz and moganite. Silica is found in nature as the mineral quartz, and its polymorphs. Taconite tailings are mostly the mineral quartz, which is chemically inert. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Rocks containing quartz (silica in composition) are silica-oversaturated. Both are stable with the common mineral quartz.