a crystal oscillator
A crystal oscillator is an oscillator whose frequency is set not by an inductor and capacitor but by a tiny, precisely cut sliver of quartz. It is the reason a cheap digital watch keeps time to a few seconds a month, and the reason your computer, phone, and every microcontroller have a stable clock. Swap the wobbly LC tank of a Colpitts or Hartley for a quartz crystal and the frequency goes from drifting by parts in a hundred to drifting by parts in a million or better.
The crystal works because quartz is piezoelectric: squeeze it and it makes a voltage, apply a voltage and it physically deforms. A thin disc of quartz has an intensely sharp mechanical resonance, a frequency at which it loves to vibrate, like a tuning fork with an extraordinarily pure note. Electrically it behaves like an LC tank but with an effective quality factor Q in the tens or hundreds of thousands, versus maybe a hundred for a coil. The circuit is usually a Colpitts variant with the crystal standing in for the inductor. Crystals come in standard frequencies, 32.768 kHz for clocks (a power-of-two-friendly 2^15 Hz that a counter divides straight to one tick per second), and values like 8, 16, or 25 MHz for digital systems.
The payoff is precision and stability that LC simply cannot touch: typical accuracy of plus or minus 20 to 50 parts per million, and far better with temperature compensation or an oven. The honest notes: a crystal's frequency still drifts slightly with temperature and ages a little over years; the small loading capacitors around it must match the crystal's specified load or the frequency shifts; and crystals can be slow to start up, taking a few milliseconds to build to full swing. But for keeping accurate time and a clean clock, nothing affordable beats quartz.
A 32.768 kHz watch crystal is chosen because 32768 equals 2^15. A 15-stage binary counter divides it down to exactly 1 Hz, one tick per second, driving the watch's seconds hand. Two roughly 12 picofarad load capacitors tune it to its specified frequency.
Quartz's ultra-sharp mechanical resonance gives a clock far steadier than any LC tank.
A crystal is precise, not perfect: it still drifts with temperature (a few ppm) and ages over years, and the wrong load capacitance pulls its frequency off target. It also has two nearby resonances (series and parallel) and must be operated at the one the circuit was designed for.