Text: A general radio term for a mineral used as a detector of radio signals. [These detectable radio signals or electromagnetic waves permeate everything, everywhere.] The physicist F. Braun noted in 1874 that certain pairs of crystals when arranged so that only a small area of surface was in contact, offered high resistance to the passage of currents in one direction while permitting them to pass readily in the other. This was really the birth of the crystal detector, although radio was not then developed. There are a great many different minerals that will act as detectors for radio signals, some of which operate with a wire, or other metallic contact, others which require a combination of minerals, one in contact with the other. There are also numerous synthetic crystals such as carborundum. Then again some of the more common natural crystals are subjected to treatment by heat, or other means, to change their nature or to make them more sensitive. The following crystals are most used in radio reception; galena, silicon, molybenum, carborundum, zincite, bornite, chalcopyrite and cerusite.

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Source: 190