Kryptonite

Kryptonite is a trans-uranium element (atomic number #137) suspended in a tellurium-silica crystal lattice. It is an extremely rare element naturally occurring only in a few places in the universe. Notably it was present in the solar system of the planet Krypton.

Kryptonite absorbs and gradually re-admits trapped Gamma Rays. These energetic discharges are not wholly benign but brief exposure is generally harmless to most sapient life forms. Kryptonian biology is extremely sensitive to these particles once their properties are altered by the molecular structure of Kryptonite. Their cells absorb the particles directly into their mitochondria, which drains the the energies that fuel a Kryptonian. Ten minutes of exposure is usually fatal; recovery takes approximately ten minutes per minute of exposure to regain full capacity. Kherans have a weaker reaction to Kryptonite but even brief exposure is similar to a carcinogenic substance.