Green Kryptonite

History
Green Kryptonite is a radioactive element composed of pieces of the exploded planet Krypton. Green K is the most common variety of Kryptonite but even it is rare. Very little Kryptonite has landed on earth.

Although harmless enough to humans it can be devastating to surving Kryptonians. Just being near the rock weakens them and prolonged exposure could reult in fatal radiation poisoning.