|MadSci Network: Chemistry|
In theory, you could make diamond from coal, but it is very difficult and the diamonds that you made would be tiny and not much use for anything. Diamond is a form of pure carbon and natural diamonds are formed very deep inside the earth, when carbon is exposed to very high pressures and temperatures. The other main form of carbon is called graphite - it is the same stuff that your pencil leads are made from.
Artifical diamonds are made in industry from graphite because it is a very
pure form of carbon. You need temperatures of over 1000 degrees Celsius
and many thousands of atmospheres pressure for this process, so it is very
expensive. Coal is not pure carbon, so the process would not work so well.
I think this link at Wikipedia will help explain diamonds to you:
Try the links in the MadSci Library for more information on Chemistry.