So I know lightsabers are made of kyber crystals, but how are they made exactly?
What is the process of making a kyber crystal into a killing machine?
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As per this article:
A lightsaber is made up of four (technically five) key components from top to bottom: an emitter, one or two sleeves, a switch and a pommel. The emitter is where the blade comes out, the sleeves are usually grips, the switch turns the saber on, and the pommel adorns the bottom. The last piece is a kyber crystal, the power source of the saber and what gives each one its color.
A special attunement of the Kyber crystal must be done in the current canon. Season 5 Episode 4, "The Gathering," from Star Wars: The Clone Wars depicts Yoda and Ahsoka taking a group of younglings to retrieve their crystals and build their sabers.
The four-episode arc is a must-watch for fans, and while it does explain that lightsabers are essentially made of curated junk, it shows that one's first saber must be made using force levitation. The components need to "belong" together and building incorrectly can be quite dangerous. However, aside from the mechanical pieces, the only thing that is missing is a proper crystal. This is where things get more spiritual than mechanical. You can't just use any kyber crystal-- it has to choose you.
Jedi call the process "bonding" with the crystal. A compatible crystal shines and glimmers, but only to the person it chooses. Once a Jedi bonds with a crystal through meditation, the crystal changes color to match the nature of the person it chose. So, while older non-canon stories attribute meanings to different colors, such as levels of experience and Force sensitivity, the real explanation is simple-- the crystal's color matches the essence of the person it bonds with. Some colors are rarer than others, like purple and yellow, but red cannot be attained naturally at all. For a crystal to be red, it has to bleed.
According to Star Wars lore, kyber crystals are naturally charged with light force energy and resistant to dark. Red crystals used to be synthetic, but in the new canon, they're made from natural crystals like the others. For dark side users to activate one, it has to be bled-- a process similar to bonding, but instead of making a connection with the crystal using the Force, it has to commanded to activate through dark side corruption (or angry meditation, basically). Once bled, a crystal turns a bright red, which is why all Sith lightsabers match. It's not an angsty aesthetic thing-- it's because of an ancient dark side ritual.
As Valorum noted in the comments, the episode also shows that the crystals the Younglings fetch seem to already be of the size and shape necessary for use, requiring no additional shaping.
If you're more of a visual learner: