The Darklight Sith use the Heart of the Guardian crystal (Bronze with a yellow core) in their lightsabers. The Heart of the Guardian is an ancient crystal which has separate beliefs from the Jedi and Sith both. It is believed that it was involved in the founding of the Jedi Order. However, it came out to be part of the Sith's heritage instead. According to prophecies of the Jedi Order, the Heart would appear at the time of greatest turmoil and help in bringing the galaxy into salvation. The Sith believed it to be an object that would have brought about their domination of known space, which wasn't the case.