A crystal oscillator, or system crystal, is a part of a computer. The computer uses it like a clock to keep track of time. It makes use of the mechanical resonance of the vibrating crystal, which has piezoelectric properties, in order to obtain an electric signal with a high-precision frequency. Crystal oscillators are considered superior to ceramic resonators as they have higher stability, higher quality, lower cost and are smaller in size.
These oscillators are usually made of Quartz crystal, even though other substances like Rochelle salt and Tourmaline exhibit the piezoelectric effect because, quartz is inexpensive, naturally-available and mechanically-strong when compared to others.
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