Coal is formed from heat and pressure, just like diamonds are. However, diamonds require much greater temperatures and WAY more direct pressure to form, which is why the end result is so different. The extreme heat and pressure can only be found far into the earth. Since coal is formed near the surface, the heat and pressure are far less severe. Diamonds require temperatures of about 2200 degrees Fahrenheit, and pressure of about 725,000 pounds per square inch. It the extreme heat and pressure, followed by cooling, which gives diamonds a hardness not found in coal or any other substance. The cooling takes place when the diamonds are forced to the surface by the volcanic eruption or earthquakes.