Diamonds may be cooked to significantly greater temperatures in the absence of oxygen. Diamond crystals turn into graphite when heated above the temperatures specified below. Diamond's ultimate melting point is around 4,027° Celsius (7,280° Fahrenheit). However, it can be melted at pressures lower than that required for other materials, such as silicon or germanium. The amount of energy needed to melt diamonds varies depending on how they are measured and what type of material they are made of. The average mass of a single-cut diamond is about 0.20 grams (0.0008 oz.). To melt this weight of diamond, an input of 7.6 megajoules (mj) is required. This is equivalent to heating it by burning 1 kg (2.2 lbs) of coal for a whole year straight! The calculated value has been shown to be too low, however, because it does not take into account other factors that would increase the required energy, such as pressure or the size of the diamond. It has been estimated that to melt all the diamond in the world would require more energy than the planet possesses available as thermal energy. According to one study, the total global energy supply is enough to melt only half of the known diamond reserves we have today.