Stygium

Stygium as a Cloaking Device
Stygium was a crystal only found on Aeten II. The Galactic Republic, in 22 BBY, passed a law controlling the trade of stygium, as it was a crystal that, when used properly, could entirely cloak a vehicle. In the Galactic Republic's early years these devices were common, since Aeten II held an abundance of them. These vehicles included the Scimitar, Darth Maul's flagship, which carried an array of stygium crystals in its long prow. In the Star Wars trilogy IV to VI, Grand Admiral Martio Batch sent an Empire superweapon to Aeten II, the Tarkin. There, the Tarkin blew open the planet to reveal its stygium core. Using the crystals and the original stygium implementation, he started the Phantom TIE project. However, saboteurs from the Rebel Alliance destroyed the project and Batch fled. The stygium crystal cloakings disappeared after the creation of the New Republic, although they were still available from Aeten II.

Stygium as a Lightsaber Crystal
It is not currently known how stygium would be used as a lightsaber crystal and its possible enhancing effects. Should you know anything on the subject, add it to this article to improve the article.

Appearances
Episode I: The Phantom Menace