With the introduction of Ilum as a visitable planet, a discussion of lightsaber crystals seems most appropriate. Although (as Luke Skywalker says in the Young Jedi Knights series) "lightsabers are powerful weapons [and] their design is so flexible that practically any kind of crystal can be used," Jedi preferred the Adegan crystals from the Ilum system. These crystals produced blue or green lightsaber blades and came in three general quality types. The Kathracite crystals were often used in training sabers since their focus did not produce a powerful blade. The most common crystals used by Jedi were the Mephite crystals, since they were high enough quality to produce a cool enough blade as to not short out the saber, and they were common enough in the Ilum cache that every Jedi could have one. If she were lucky enough, a Jedi would find a Pontite crystal. These had the strongest Force aura, so they focused well, giving them formidable strength and intensity.
Although it is possible for any Jedi to wield any lightsaber regardless of crystal color, it was traditional for specific types of Jedi to carry certain colors. For instance, a Jedi Guardian would carry a blue-bladed saber. Anakin Skywalker, Obi-Wan Kenobi, and Plo Koon are examples of this. However, other Guardians like Qui-Gon Jinn and Agen Kolar did carry green blades. If this had to do with their propensity for the meta-physical side of the Force, it is unknown. Maybe it brought out the color of their eyes.
The green blade was generally reserved for Force users who lean more on powers than lightsaber prowess. I have my own theory that the green crystals were actually less powerful than the blue crystals, so Jedi would reserve blue crystals for those who pursued lightsaber skills. However, Consulars focused on telekinesis and healing in battle, so it wasn't exactly necessary for them to have the most powerful lightsaber.
Synth-crystals were thought to be less useful than the naturally occurring crystals like the Adegan crystals. Most could not even be used in lightsabers, but if augmented through the Dark Side of the Force these crystals could be made more powerful than any of the Jedi crystals. The Sith were known to be masters of cellular manipulation as with Sith Alchemy. This sorcery held that the Force has ability to change the chemical make-up of minerals, plants, even intelligent life forms. Darth Plagueis believed that it could even create life.
When a syth-crystal was imbued with the hate and anger of a Dark Force-wielder during the creation process, it would turn red inside the geological compressor. The compressor emulated the geothermic conditions of any world. Unfortunately, this process has caused some crystals to work in lightsabers but remain highly unstable.
There are many other colors of lightsabers, including black, white, and orange, but I would like to touch on a couple of other colors that are more predominate in galactic history: yellow and purple.
Next to green and blue, yellow seems to be the most common Jedi saber color. This color traditionally was left for the Jedi Sentinel, who is said to have mastered the balance of force prowess and physical abilities. The yellow blade sets him apart from the other Jedi. At the same time, a yellow-gold color was called the Heart of the Guardian. So it would seem that Guardians may also be prone to use a yellow crystal. And you can't forget that Kenner released the first Obi-Wan action figure with a yellow saber, but I'm sure that's not canon.
Mace Windu was the only Jedi in the movies shown to break the Jedi blue-green tradition. His crystal was not an Adegan crystal. He made a special trip to the Hurikane system in the depths of Wild Space to snag one of these violet crystals. It is said that these crystals have a natural ability to pierce defenses. Since the Sith are said to have suddenly appeared from unknown regions, it is possible that is why a Sith inquisitor is seen with a purple saber blade in SWTOR's combat video documentary.
Whichever blade color you choose, it should reflect the heart of your character. Remember the Jedi crystal mantra:
The crystal is the heart of the blade.
The heart is the crystal of the Jedi.
The Jedi is the crystal of the Force.
The Force is the blade of the heart.
All are intertwined.
The crystal, the blade, the Jedi.
You are one.
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