the quartz crystal

The quartz crystal is the precise little component at the heart of a crystal oscillator: a thin wafer of quartz with metal plates on its faces, sealed in a small metal can or surface-mount package, usually showing a frequency like 16.000 stamped on it. By itself it does nothing; it is not a power source. It is a mechanical tuning fork dressed up as an electrical part, whose job is to ring at one frequency far more sharply than any coil-and-capacitor could.

Its magic is piezoelectricity. Apply a voltage across the quartz and it physically flexes; let it flex and it generates a voltage. So an electrical signal and a mechanical vibration become two views of the same thing, and the quartz vibrates fiercely at its natural mechanical resonance, set by how the wafer is cut and how thick it is. Electrically the crystal looks like an equivalent circuit of a very large inductance, a tiny capacitance, and a small resistance, all in series (with a small parallel capacitance from the plates). This combination gives an astonishingly high quality factor Q, often 10,000 to over 100,000, meaning the resonance is razor-sharp and very stable. A high Q is exactly what makes the resulting oscillator's frequency hard to budge.

Two practical truths matter. First, a crystal has two close resonances: a series resonance where its impedance dips low, and a parallel (anti-) resonance a touch higher where it peaks; a circuit is designed for one or the other, and the load capacitance you place around it fine-tunes the exact frequency. Second, quartz is extraordinary but not magic: its frequency still shifts a few parts per million with temperature (the cut, like the common AT-cut, is chosen to minimize this near room temperature), and it ages slightly over years. For better than ppm stability, designers add temperature compensation or put the crystal in a tiny heated oven.