Purification, in essence, was the act of healing a corrupted kyber crystal, freeing it from the dark side's corruption. This was the opposite of the bleeding process. Typically, a kyber crystal that had undergone this cleansing would shift in color from red to white, and would establish a strong connection with the Force-sensitive individual who carried out the ritual.
To purify a kyber crystal tainted by the dark side, a Force-sensitive being had to forge a bond, not only with the crystal itself, but also with the darksider who had originally bled it. The key was to empathize with the darksider, gaining a deep understanding of the circumstances that led them to embrace the dark side. Only then could the damage inflicted upon the crystal be reversed. This involved entering a deep, trance-like state to communicate with the crystal, learning its history and the history of the one who corrupted it. Once this was done, the healer would attempt to infuse the crystal with positive emotions like joy and hope, effectively helping it "unlearn" the pain inflicted by its previous owner. A successful purification resulted in the healer emerging profoundly changed, and the crystal, once red, would radiate a brilliant white light.
However, much like a kyber crystal could resist the bleeding process, a corrupted crystal could also defend itself against attempts at purification. It could project dark-side visions and hallucinations into the mind of the prospective savior, destabilizing them and causing them to fail, leaving the crystal still corrupted.
Following the defeat of the Sith, the galaxy was filled with bled kyber crystals, remnants from the lightsabers of fallen Dark Lords and their followers. To address this issue, the Fallanassi refined and mastered the art of purification. This light-side-aligned religious group maintained a long-standing agreement with the Jedi Order, offering to purify any kyber crystals corrupted by the Sith that they encountered. Furthermore, the Fallanassi preserved the knowledge of the cleansing ritual, passing it down through generations.
The Togruta Jedi Master Jora Malli discovered an ancient Sith lightspear during her career. Driven by curiosity about the purification process, Malli extracted the crimson crystal from the spear and purified it, purely as an academic pursuit. Surprisingly, Malli found that the newly cleansed crystal had developed a strong connection with her, and she chose to incorporate it into her own lightsaber, resulting in a white blade.
During the New Republic Era, after Luke Skywalker aided a woman named Komat in escaping the Acolytes of the Beyond—a group she was affiliated with—he purified the kyber crystal from her lightsaber, causing the blade to turn white, symbolizing the removal of evil from her mind and soul.
The concept of purification was first introduced in the 2016 novel Ahsoka, authored by E. K. Johnston; however, the term "purification" itself wasn't coined until the 2021 novel The High Republic: Light of the Jedi, penned by Charles Soule. Its initial visual representation occurred in "Aau's Song," an episode within the second volume of the non-canon anthology series Star Wars: Visions, which premiered on May 4, 2023.
Within the Star Wars Legends continuity, Jaden Korr transformed a red synthetic lightsaber crystal into a yellow one. He then used this crystal for his third lightsaber. This process was referred to as cleansing.
In the episode "Aau's Song," Aau, a Force-sensitive member of the Korban species, purifies an entire mountain of bled kyber crystals. She achieves this by singing to them, using the beauty and harmony of her voice to expel the dark side from the crystals. Consistent with canon, this process resulted in the crystals changing from crimson to white.