It is fascinating that the same element (carbon) has diametrically different properties only because of the different composition of the crystal lattice. But because there are the same atoms in coal, a pencil lead or a diamond, people wondered after the discovery of the structure of these substances (late 18th century) if a diamond could not be made artificially. Diamonds are thought to be formed at high temperatures (around 1,000 °C) and pressures (billions of Pascals, 10,000 times more than atmospheric pressure). Therefore, if we had pure carbon where we compress each atom, it should not be a problem to produce a diamond which H. Moissan managed to do at the end of the 19th century. It is complicated to achieve such high pressure and especially high carbon purity. Any additive would disrupt/alter the produced crystal lattice and therefore the artificial diamond may not have the desired properties. Reaching a temperature of 1,000 °C is not a major problem at home, it is more complicated with the pressure at which we will produce the diamond and, in my opinion, especially in the purity of the carbon. Classic coal is mainly composed of carbon, but there are other additives in it. We would also have to extract carbon from the coal in some way. This is probably why a pencil lead was used (and perhaps is still used) in the production of artificial diamonds. The production of artificial diamonds is well managed in the industry and artificial diamonds have a wide range of uses but we probably will not succeed at home… And with gold, it would be even more difficult.