Home-grown diamond using CVD
The easiest way to grow diamond through chemical vapor deposition (CVD) is on a heated filament, which just takes a few watts. 

You'd need a low-pressure chamber (~30 torr, or ~0.04 atm) with gas-tight fittings for electricity, feed-gas of ~1% $\ce{CH4}$ and ~99% $\ce{H2}$ and $\ce{Cu}$, $\ce{Ti}$, $\ce{W}$ or $\ce{Ta}$ filaments. Since this is a flow-though reactor, you'll also need to dispose of the flammable feed-gas without letting it accumulate near the vacuum pump.

N.B. This is not a simple project one could do at home, since you'd need vacuum equipment, pressure regulators, high-pressure bottles* of these highly flammable gases, and a vacuum-vessel which could implode. If you can get a knowledgeable instructor involved who has access to laboratory facilities and a thorough understanding of safety procedures, though, it's not impossible to accomplish.  

* One of the greatest dangers would be a gas bottle falling, taking off like a rocket and leaving explosive gas in its wake!