Synthetic crystal (Page 2)
It is common for the Sith to use the Force to channel their hatred for the Jedi during the forging process, imbuing it with the dark side of the Force and shaping it into greater potency.
Some Jedi also create synth-crystals, most notably Luke Skywalker and Jaina Solo. These usually have colors other than red, presumably because of the Sith or dark side connotation of a red-bladed lightsaber. Skywalker created a green crystal, while Solo produced a violet one. In fact, because Sidious had most of the sites, where crystals occurred naturally, looted or destroyed, the New Jedi Order had to rely on synth-crystals almost exclusively.
Not all red lightsaber crystals are synthetic. Red crystals are known to form naturally in the Crystal Caves on Dantooine, along with blue, green, yellow and violet crystals. However, unlike these colors, red crystals are typically found in unhatched kinrath eggs.