A quartz clock is an electronic oscillator that uses a quartz filter to achieve very precise frequencies. Usually some form of digital logic counts these cycles and provides a numeric time display that a human can read.
Chemically, quartz is a compound called silicon dioxide. When a crystal of quartz is properly cut and mounted, it can be made to bend in an electric field. When the field is removed, the quartz will generate an electric field as it returns to its previous shape. This property is known as piezoelectricity.
Many materials can be formed into plates that will resonate. However, since quartz can be directly driven by an electric signal, no additional speaker or microphone is required.
Quartz has the further advantage that it does not change size much as temperature changes. Fused quartz is often used for laboratory equipment that must not change shape as the temperature changes. This means that a quartz plate's size will not change much with temperature. Therefore, the resonant frequency of the plate, which depends on the plate's size, will not change much, either. This means that a quartz clock will be relatively accurate as the temperature changes.
In modern quartz clocks, the resonator is a tuning fork shape, laser-trimmed or precision lapped to vibrate at 32,768 cycles per second. In most clocks, the resonator is in a small can or flat package, about 4mm long. Standard quality resonators of this type are warranted to have a long-term accuracy of about 6 parts per million at 31°C, that is, a typical quartz wristwatch will gain or lose less than a half second per day at body temperature.
The relative stability of the resonator and its driving circuit is much better than its absolute accuracy. If a quartz wristwatch is "rated" by measuring it against an atomic clock's time broadcast, and worn on one's body to keep its temperature constant, the corrected time can easily be as accurate as 2 seconds per month, more than good enough to perform celestial navigation.
The rest of the clock is usually a small, special-purpose computer with a program that counts the cycles, and translates them into an electrical form to drive the visible display. The use of computers in electronic timepieces has enabled a wealth of features, such as stop watches, perpetual calendars, multiple perpetual alarms that play tunes, and other features that would be impractical with simple electronic counters.
Some premium clock designs self-rate. That is, rather than just counting vibrations, their computer program takes the simple count, and scales it using a ratio calculated between an epoch set at the factory, and the most recent time the clock was set. These clocks usually have special instructions for changing the battery (the counter must not be permitted to stop), and become more accurate as they grow older.
It is theoretically possible for a computerized clock to measure its temperature, and adjust for that as well.
Quartz chronometers designed as time standards often include a crystal oven, to keep the crystal at a constant temperature. Some self-rate and include "crystal farms," so that the clock can take the centroid of a set of time measurements.
The inherent accuracy and low cost of production has resulted in the proliferation of quartz clocks and watches since the 1970s. Quartz timepiece production has emerged from Asia, notably Hong Kong and Japan. Many traditional European clockmakers have continued to produce the less accurate but popular geared timepieces.
When buying a quartz watch, unless it has special features, above US$20, you are paying for the packaging: it will not be a better timekeeper.
See Also: Quartz Oscillator.