Kryptonite

Kryptonite is an extremely rare mineral naturally occurring only in a few places in the universe. Notably it was present in the solar system of the planet Krypton. The ore is shot through with a trans-uranium element (Krypton, atomic number #36) suspended in a tellurium-silica crystal lattice.

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. The byproduct Alpha particle emissions can cause cancer in humans over prolonged exposure periods.

Kryptonian biology is extremely sensitive to Gamma Ray particles that have passed through Kryptonite. Their cells absorb the energy directly into their cellular 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.