No. Coal is full of rubbish and would just give you a hard black mineral. To make diamonds, you need perfectly pure carbon - probably in the form of graphite - and then subject this to several thousand atmospheres of pressure and temperatures beyond imagination. All this activity pushes the carbon crystals in graphite into the compacted crystalline structure that is diamond. The exciting thing is that we are doing exactly that right now!!! Perfect gem-quality diamonds are being made right now, although I hear they are mostly yellow in color Next step is to produce flawless diamond in sheets so that the computer wallahs can make farcically fast new supercomputers.