Scientists have used a new technique to synthesize diamonds at normal, atmospheric pressure and without a starter gem, which could make the precious gemstones easier to grow in the lab.
Either JPL or LM (I can't remember which) was working on a HTHP system with the goal of being able to grow diamonds with ICs built in. I wonder if this has that potential.
Carbon, compressed hard enough it becomes diamond, forms a square crystal lattice that is so crazy stable that it's very resistant to change (including from burning).
I don't know specific temps, but graphite forms a hexagonal lattice that is highly stable on 2 dimensions. The bonds between layers of hexagons are weaker which lets it be brittle in all the ways we love graphite. I'm not sure how this specific geometry changes the affinity to bond to oxygen.