A Synthetic lightsaber crystal, commonly referred to as a Synth-crystal, is a type of lightsaber crystal that is artificially created, rather than naturally formed by geological processes. Due to the methods used in their creation, they often formed with a red coloration, though they could be made with any color through special manipulations of either the creation process or the crystal itself, often through the Force. The red-hued lightsaber blades generated by synthetic crystals with a red coloration were often nicknamed "bloodshine blades". As a result of their artificial origins, synthetic crystals created more powerful lightsaber blades and could be more easily augmented.
For more information read Wookieepedia's Synthetic lightsaber crystal article.