Ruusan crystals came from the planet that bore the same name. Prior to the rise of Sith Lord Kaan's Brotherhood of Darkness, Jedi Knights favored Ruusan crystals for their ability to help one focus on the Force.
After the Battle of Ruusan and the defeat of the Jedi and Sith forces, both the Jedi Order and the Republic made an effort to forget Ruusan's existence. With the Valley of the Jedi harboring an immense amount of power, the Jedi felt it necessary to protect that Force power from every being used for evil deeds. As such, Ruusan eventually fell into legend, its location lost to time. Ruusan crystals became extremely rare in the galaxy passing out of all knowledge as well.
Millennia later, a Dark Jedi Master by the name of Jerec sought the location to the Valley of the Jedi in order to consume its power. A young Jedi Knight by the name of Kyle Katarn tracked Jerec to Ruusan, defeating his seven minions, Dark Jedi themselves, along the way.
Jedi Katarn ultimately defeated Jerec in the Valley of the Jedi freeing the Force spirits trapped there. With Ruusan location discovered and its threat to the galaxy eliminated, the possibility emerged for Jedi Knights of Luke Skywalker's new Jedi Order to return to Ruusan and once again wield lightsabers bearing the Ruusan crystal.