Quartz crystals are formed by something called the “"hydrothermal process.” The word, if broken down describes “hydro = water” and “thermal = temperature.” Thus Quartz crystals form under conditions of high temperature, in the presence of water under high pressure. There also needs to be a mineralizing solvent present in the water to help it to dissolve more Silica. In the earth what happens is that an area deep beneith the surface that is rich in brackish water and silica rock, is exposed to heat. This heat increases beyond the boiling point of water, however, the intense pressure of overlying rock prevents it from turning to steam. Instead, it becomes a powerful solvent capable of dissolving the silica in the rock. What usually is thought to happen is that the heat source begins to cool, or the supersaturated solution of silica moves away from the heat. In either case, the liquid can no longer hold as much silica in solution, so it begins to crystallize. Depending on how much of this silica rich water was formed, and how much room there is for the crystalization to grow, crystals of Quartz begin to form.
