I know that artificial diamond may be made in a special press with heat and pressure. This process yields expensive and small crystals.
Is there another way?
Suggestion of a synthesis route:
Get some pure Iron
Get quantity of carbon
Heat iron to white heat with a reducing flame
Add carbon powder to white hot iron in reducing flame until no more is absorbed
Allow Iron to cool (Quickly by Quenching? or Slowly in Sand?)
Dissolve Iron in dilute sulfuric acid.
Filter liquid and collect large diamonds?
I have no idea if this suggestion will work - its something I "remember" doing as a child in the kitchen!!!!