Polycrystalline quartz was formed from amorphous silica in seawater and distilled water at temperatures and pressures ranging from 50 to 450°C and 50 MPa to 3 GPa, respectively. The data at 100 and 200 MPa pressures can be reasonably modeled using Avrami's nucleation and growth equation. Experimental results reveal a dramatic pressure effect on transformation rate, which increases approximately by five orders of magnitude as pressure increases from 50 MPa to 3 GPa at 400°C. The effect is more prominent in a low-pressure regime than in a high-pressure regime; the transition pressure between the high-and low-pressure regimes is temperature-dependent. The pressure can significantly enhance both nucleation and growth rates although the effect is stronger on nucleation than on growth. This finding leads to the successful synthesis of polycrystalline quartz at a temperature as low as 50°C at 3 GPa in a few days, and helps predict its formation at ambient temperature and higher pressures. SEM measurements show that grain sizes decrease significantly with increasing pressure, giving rise to a new method for synthesizing nano-size (< 100 nm) polycrystalline quartz in neutral solutions.