Diamagnetism is a type of magnetism. They very weakly repel magnets. Most substances are diamagnetic. It was discovered in 1778 when someone saw that bismuth and antimony repelled a magnet. Diamagnetic things can be levitated. A frog was levitated in a very strong magnet. Water can be repelled by a magnet because it is diamagnetic. It is too weak to be noticed normally, though.