Synthetic diamonds are made under very high pressure and heat, this seems to bypass both. I will read the article later when I am on something bigger than my phone but synthetic diamonds production mimics natural diamond creation process.
Yes, and there’s a reason for that - high pressure and heat - because carbon does not naturally like to be in the state of “diamond” (otherwise coal mines would be known as diamond mines). No simple 3D printing process is going to get around that unfortunate fact. I.e., a 3D printing process in itself does not have the energy to turn coal (or some other carbon-containing powders) into diamond.
When I was a kid superman used to squeeze coal with his bare hands and make diamonds.