Why does it require so much energy/pressure to create diamonds, yet with silicon it is very easy and (I believe) naturally occurs in a "diamond" structure, i.e. where each atom is covalently bonded to four adjacent atoms? I guess it has something to do with the fact that the outer shell is further away from the nuclear, since that is the only significant difference between the two atoms, but I can't work out why that should matter here.
There are two allotropes of carbon crystal, graphite and diamond. Both are stable, so the formation of diamond structure is competitive with the other structure (or structures, if you include buckyballs and tubes). The important consideration, therefore, is crystal growth rate, which will depend on solvent and seed material, as well as temperature and pressure.
For manufacture of large artificial diamonds, liquid iron 'solvent' is used, which is why high temperature is important; carbon has to diffuse through the liquid solvent. At high temperature, the (relative) instability of diamond means that a pressurized environment is required: diamond has to expand (diamond is 3.53 g/cc, but graphite is 2.23 g/cc) in order to become graphite, and pressure prevents that. Pressure alone might become so high that graphite recrystallizes as denser diamond, in a shock wave (as long as you only want to make small diamonds) made with explosives. At elevated temperatures and low pressure, diamond recrystallizes as graphite, however.
It is possible to grow diamond films (depending on a substrate that seeds the crystal form of interest) at low pressures as well, from vapor. Pyrolyzing CH4 in the presence of a cold surface that doesn't seed the graphite form is the general approach.
To summarize: in growth of a carbon crystal, the environment determines the dominant crystal structure in a number of ways. The Diamond Makers by Robert Hazen, is a good treatment of this subject.