Although piezoelectric effects were known in the late 1800s, the crystal oscillator was not invented until 1917, when Alexander Nicholson developed a working concept for Bell Laboratories. Several other pioneers worked independently with the idea, creating oscillator models of their own design.
Older electronic designs depended upon less stable components. Resistor-, capacitor- and inductor-based oscillators needed constant adjustment to remain on frequency in radio applications.
Ordinary rock crystal resonates when triggered by electronic white noise. When a voltage is applied, the crystal physically distorts; when removed, the crystal emits an electrical signal.
When part of the output signal of the crystal is returned and used as the triggering source, a feedback loop is formed. With sufficient energy applied to the circuit, the ringing becomes self-sustained or resonant.
Physical shape and size of the crystal slab determines the fundamental frequency of its oscillation. External components can slightly tune the circuit away from the fundamental for precise tuning control.