In Legends. Not directly a material but metal enhanced by Sith Alchemy
Sith swords were constructed through various metallurgical and arcane means, transmuting base metals into an alchemical alloy possessing an unusual affinity for dark side energies. The rare metallic compounds used in its construction could result in blades varying in weight from 3 kilograms up to 6.5 kilograms, a blade far heavier than its 1.4 meter length[source?] would indicate. It often required great strength of body as well as in the Force to successfully wield the weapon and make it as dangerous as a lightsaber. In fact, the alchemical alterations allowed the Sith sword to block incoming lightsaber attacks, as well as deflect and redirect blaster bolts in much the same way a lightsaber did.
From the Sith Swords article on Wookiepedia