Since diamonds are made with coal under pressure, why don't we turn mountains of coal into diamonds?

Not clear to me that diamonds come primarily from coal (13C compositions are not consistent with that idea), but it does not matter because we can and do already make synthetic diamonds.  We simply use a more pure starting material than coal.  Coal is not a very pure substance, and impurities are a problem for the process.

Coal is just another type of carbon. Humans are another source of carbon and the ashes from humans can also be made into diamonds.

There is a cost of heat and pressure to make them, and the electrical energy to make the heat and pressure!

1 we burn the coal to make steel and electricity

2 turning coal into (industrial) diamond costs a lot of money and energy

It works in theory, but it is a lot more cost effective to mine the diamonds

Because it would take enormous amounts of energy to do that.

Cost (15 characters)