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.
I have been doing a little research on this process as bonding quartz cost effectively is something I have been working on for years.
Current 3D-printers are operating at a extremely much coarser level than this. To 3d-print a diamond you’d have to position individual atoms of carbon in a lattice,
The diamond phase C-C bond requires extremely high pressure and temperature to form and, in their absence, what you will get from 3D printing with a diamond suspension or a diamond paste will be a “green” preform, where the diamond grains will be loosely held by a binder (a glue). Such a material, although consisting of many tiny diamonds, it is not considered “a diamond” as there is no material continuity within it.
It is however quite possible to 3D print a diamond-in-metal-matrix composite, like those used for tools. It has not been applied yet, though, as such material is expensive.
https://3dprint.com/146562/diamond-3d-printer-lockheed/
Lockheed Martin, aerospace and defense corporation and big name in metal 3D printing, has filed a patent application for a 3D printer that prints diamonds. Yes, they’re synthetic diamonds, but diamonds all the same. Diamonds are formed when the extreme heat and pressure in the earth’s core cause carbon atoms to bond together and form super-hard crystals. Lockheed Martin’s diamond printer uses poly(hydridocarbyne), a carbon-based pre-ceramic polymer with structural similarities to diamonds.
That polymer is deposited in layers, alternating with layers of a ceramic powder, by the printer in a predetermined shape. The deposited shape is then heated to temperatures over 100 degrees in an inert atmosphere to cause pyrolysis, a physical and chemical change that happens when an organic material is heated without the presence of oxygen. In this case, the heat causes the poly(hydridocarbyne) to crystallize into a diamond. Any excess ceramic powder is then removed.
That’s one proposal. The patent application also discusses the use of other pre-ceramic polymers; another proposal involves the use of nanodiamond powder.