AbstractLarge pressure gradients and unrecognizable variations of pressure occur when solid substances are compressee by cold anvils (pistons) and heated by internal heaters. To improve the reliability of the results the miniaturization of samples and preliminary firing of pyrophyllite (which transmits pressure) are suggested. The equilibrium coesite—stishovite may be empirically expressed as:Pkb= 85 + 0·364 T (°C)Stishovite must be playing an important role in the mantle at depths greater than 500 km. Quartz and stishovite do not coexist in nature whatever PT‐condition