Natural diamonds aren't always the best choice for scientific and industrial applications, so scientists developed several ways to grow them under controlled conditions. One method uses a High Pressure High Temperature press.
This process duplicates the high temperatures and pressures needed for a diamond crystal to form. Above is a basic diagram of the BELT type hydraulic press. Other types of presses exist, as well, but this is one of the most prevalent models in use for diamond production.
A diamond seed is placed at the bottom of the press. Then, a tubular, graphite heater generates temperatures above 1400°C and melts the solvent metal, usually nickel or iron. The molten metal dissolves the high purity carbon source, which is then transported to the diamond seed where it forms a diamond crystal. The addition or subtraction of certain trace elements produces different colors. A diamond crystal of about a carat in size can be produced in as few as three days.
Nitrogen causes yellow, the elimination of nitrogen produces a colorless crystal, and adding boron makes blue. Other colors are possibe, too. Fancy yellows are the easiest, as nitrogen is plentiful in the atmosphere and it can be very difficult to eliminate it from the process.
Rather than spending their efforts on producing colorless crystals for the jewelry industry, most producers focus on reproducing the fancy colors. Fancy colors are much more rare in nature than the colorless and near colorless varieties. A blue, yellow, or pink synthetic diamond is well within the reach of the average consumer, as opposed to their natural counterparts.
Synthetic diamonds can display some very interesting fluorescent colors and patterns under short wave UV. Natural colorless diamonds usually exhibit an even, white or blue when they do fluoresce.
Here is an example of a synthetic diamond fluorescing green:
This one shows off the growth pattern of the crystal:
Advances have been made to minimize characteristic fluorescent patterns in synthetics co that they more closely mimic their natural counterparts.
Synthetic diamonds of high quality are used in electronics for their excellent heat transferring abilities. The processor in your computer may use one as part of its heat sink. I have also seen diamond slices used as scalpel blades.