X-Kryptonite is a type of Kryptonite, created by Supergirl when she was conducting scientific experiments to try and cure herself and Superman of their weakness to Kryptonite. When she failed, she discarded the meteor, and it gave Streaky his superpowers, temporarily. ("Supergirl's Super Pet!")
In the Superman and Lois series, X-Kryptonite is used by Morgan Edge (Tal-Rho), Clark's (Kal-El) Kryptonian half-brother, as a power source for the Eradicator. Edge used this machine (Before he became himself the Eradicator) to transfer Kryptonian consciousnesses into the bodies of the inhabitants of Smallville who are the only ones who can survive the effects of X-Kryptonite.
In the Arrowverse it is yellow-orange in color and has the appearance of a crystal. In this universe it is one of the rarest types and is only found in the mines under the town of Smallville, which explains the resistance of its inhabitants to its radiation since they have been exposed to it for more than 20 years. In the Superman DC Heroclix Miniatures Game it is dark blue.