Can you compress pure carbon into diamonds? I'm doing a science project, and we're wondering if it is possible to compress pure carbon (C) to the point where it becomes diamonds? What would the process have to be and how much energy would this take? Has this been done and is this feasible?
 A: The technique of using high pressures to make diamonds was pioneered by Francis Bundy of General Electric and others back in the 1960's using large volume presses, I believe. If you look at the carbon phase diagram, a pressure of at least 15 kbars (15,000 atmospheres) is required to turn carbon into diamond, but the transformation is very sluggish. Usually much higher pressures are used and also the carbon is heated to high temperature to accelerate the kinetics of the transformation. I believe that a catalyst like nickel is also often used. Currently, I think that Sumitomo of Japan does a lot of work in high-pressure synthesis of diamond for applications such as heat-spreaders for high-power electronics. They can actually grow rather large single crystals of a carat or so. Normally, their diamond crystals have a yellowish color, probably because of nitrogen impurities which they may be adding as a catalyst.
Nowadays, a popular method of growing synthetic diamond is not by using high pressures but by using a low pressure chemical vapor deposition (CVD) process in which diamond is grown from a methane and hydrogen plasma. In research which I perform with my colleagues, we actually use CVD diamond growth to encapsulate tiny electrical circuits inside diamond crystals to fabricate "Designer" diamond anvils for high pressure research.
A: This is not entirely serious, but if you're looking for a really great project, you can't do much better than this. Are you aware that you can make diamonds with high explosives? And since the starter mixture is not well-known, you might have to make multiple attempts.
A: Theoretically yes but those diamonds would be industrial, not worth much and wouldn't have the exterior of natural diamonds. Like what was said in a previous answer, these diamonds can be made in an explosion too! I think there's a myth busters episode on it if I'm not wrong! 
A: my profesor of structure of matter told us that is widely done in Russia, where there is a surplus of explosives and so the price is low.
The procedure consists in filling a container with graphite on the borders and with explosives in the center, and make the expolsive blow up.
This procedure will produce diamonds $100-200 \, \mu m$ thick, and not pure, so is suitable for  industrial grade diamond production. 
So if you was wondering to create a huge diamond for a ring to give to your gf at the science fair you're better choose another method ;)
