Crystal are normally required to provide clock pulses to your microcontroller or other IC's which require external clock source.
An oscillator crystal has two electrically conductive plates, with a slice or tuning fork of quartz crystal sandwiched between them. The crystal oscillator circuit sustains oscillation by taking a voltage signal from the quartz resonator, amplifying it, and feeding it back to the resonator. Quartz has the further advantage that its elastic constants and its size change in such a way that the frequency dependence on temperature can be very low.
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