Kimber stone

Kimber stones were lightsaber crystals produced by volcanic activity on the moon of Y'ness. They were once considered unsuitable for use in lightsabers because they produced a non-lethal blade. However, certain Jedi Knights believed the stones had potential, leading to a number of them being removed from Y'ness before the Jedi Order's destruction. By the time of the Empire, these crystals were sold as simple trinkets on the black market by individuals who did not understand their true nature and value.

Text and facts from the “Kimber stone/Legends” article on Wookieepedia, licensed under CC BY-SA 3.0. Modified for this site.