Discussion
The naturally occurring crystal which was used as a detector of radio waves is
Answer: B
The crystal is Galena.
Galena, natural lead sulphide (PbS), was the crystal in the early 'cat's whisker' detectors of radio receivers. A fine wire touching the crystal formed a point-contact rectifier, one of the first semiconductor devices, which demodulated the radio signal.
Silicon and germanium are semiconductors used in later diodes, but they are refined materials, not naturally occurring crystals used in that role. Ruby is the crystal of the first laser, and zinc selenide is an infrared optical material.
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