What Makes Diamonds Difficult to Produce? Having seen an answer over on Worldbuilding about very strong/dense wood that suggested artificially creating some enzymes that would manufacture diamond/graphene as the cellular binding materials in the tree, I said to myself, "Hold on, I know this won't work: creating diamonds requires high temperature and/or pressures...doesn't it?"
But I was unable to locate any information as to why this is the case: that is, what physical property of the bonds or arrangement of the carbon atoms dictates the intense pressures needed to cause the formation of the crystal lattice?  Or is there really nothing standing in the way of a chemical process (i.e. an enzyme constructing it a few atoms at a time, albeit with large energy expenditures and slow timescales) that would do it other than "we don't know how to make that."
The covenant bond energy between two carbon atoms seems pretty high, I'll admit, at 348 kJ/mol, but it's less than some other bonds, say Carbon and Hydrogen at 419 kJ/mol (source). So it doesn't seem like that's the limiting factor.  I do know that there is energy stored in the organization of the lattice itself, but I don't know how much that contributes; Wikipedia only helpfully notes that the energy is "greater in materials like diamond than sugar."
This page notes that diamonds are stable because the phase boundary has a high energy threshold (but not what causes it or how big it is) but the phase diagram lead me to the CVD Diamond and this company's FAQ again notes that the difficulty is in the lattice, but again skips over the underlying reason and the specific energy levels involved.  Then there is this question where John Rennie notes that most large molecules are difficult to make and that for carbon nano tubes specifically, it's largely due to the fact that carbon isn't soluble in anything (useful).
 A: Well, the problem with making diamond out of graphite by means of any normal chemical reaction, either with or without a catalyst, is that diamond is not even stable at room temperature and atmospheric pressure. That means that one either has to (1) change the pressure and temperature conditions so that the diamond phase of carbon does become the energetically stable phase or (2) use some sort of non-equilibrium process to make diamond out of carbon (e.g., microwave chemical vapor deposition). 
To make diamond the energetically stable phase, one has to pressurize carbon to over 100 kbars (about 100,000 atmospheres pressure), but even then the transition from graphite to diamond is very sluggish and slow. Heating up the carbon to high temperatures while it is under high pressure helps to speed up the transition a bit. Also, scientists creating diamond from graphite in high-pressure devices found that a metal (nickel, I believe) could act as a catalyst to further speed the transition. 
The alternative method of creating diamond is to use a non-equilibrium process like microwave-plasma chemical vapor deposition (MP-CVD), in which a low-pressure methane-hydrogen gas mixture in a chamber is turned into a plasma with microwaves, and then diamond crystals are grown on a substrate inside the chamber. The process chemistry is quite complicated and involves the growth of both diamond and graphite on the substrate due to the presence of the carbon-containing methane while the ionized hydrogen preferentially etches away the graphite so that the net growth of diamond is positive while the net growth of graphite is negative. Diamond growth rates of over 10 microns per hour have been achieved. 
Interestingly, the energy difference between diamond and graphite is not large. It's only about 0.03 eV per atom, but unfortunately it's in the direction of 0.03 lower energy for graphite compared to diamond so, again, diamond is not thermodynamically stable at room temperature and pressure and conventional chemistry techniques don't offer any way to make diamond from graphite.
(A final note: The technology for making diamond has been advancing by leaps and bounds in recent years. I've been involved in a research group using synthetic diamond to encapsulate electrical diagnostic circuits in diamond crystals for use in ultra-high pressure experiments to pressures up to millions of atmospheres: https://str.llnl.gov/str/December04/pdfs/12_04.1.pdf )
