Kryptonite

A fictional chemical element that was produced from the material of the planet Krypton when it was destroyed in an explosion. Kryptonite is usually found in the form of a glowing green crystal, but metallic forms have also made appearances along with different-colored variants such as red kryptonite. These different forms may represent multiple allotropes of kryptonite. Recently all known kryptonite on Earth was transmuted? into iron, but Kryptonite can be synthetically manufactured by a variety of unknown means and additional material left over from the destruction of Krypton continued to fall from space.

Kryptonite is for the most part harmless to humans, but has adverse effects on Kryptonians. See Superman for more details.

It is speculated that kryptonite may be located in a hypothetical "[island of stability]?" high on the periodic table, beyond the currently known unstable elements.