Quartz crystals produce an electric voltage, a property known as piezoelectric. Unable to understand the characteristic, ancient cultures attributed many mystical properties to quartz crystals. For thousands of years various European cultures believed that the mind of a medium became receptive to the spirit world via the influence of quartz when it was fashioned into a sphere or crystal ball. Quartz was believed to act as a psychic purifier, tuning one into their inner "vibrations". It was believed that clear crystals possessed the ability to amplify emotions, enhance concentration and intuition, and neutralize "negative energies".