CRYPTONITE fictional mineral invented by Marvel for the Superman series. In reality, it is about remains of the planet Kripton, from which Superman is originally, and which are mortal to him, so the villains always try to approach him a piece to weaken him and eventually, kill him. Superman's father Jor-El put him in a space capsule moments before the planet's collapse, which brought him to Earth, being found in the field and adopted by the Kent couple. Therefore, the correct thing is to write it Kryptonite, with K.
