Explanation: Quartz is the most widely used substance in crystal oscillators due to its strong piezoelectric properties. This means it generates a voltage when mechanically stressed and, conversely, vibrates mechanically when an electric field is applied across it. When placed in an appropriate circuit, a precisely cut quartz crystal will resonate at a very stable frequency, acting like a high-Q tuned circuit. This characteristic is crucial for generating accurate clock signals and stable carrier frequencies required for reliable radio transmitters and receivers. Galena was historically used as a semiconductor in early crystal radio detectors, not as an oscillator element. Silicon is a semiconductor fundamental to transistors and integrated circuits, but it does not exhibit the piezoelectric properties needed for crystal oscillators. Tourmaline is also piezoelectric, but it is generally more expensive and offers less performance advantage over quartz for most common oscillator applications.