What is special about these elements that makes them good for clocks?
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The quartz crystals used are piezoelectric crystals. There are crystals, rocks, which produce an electric current when you bend it, and similarly will bend when you apply an electric current to them. So you can hook it directly up to an electric circuit without any additional interfaces. This makes them easy to use as pendulums in electric clocks. A wafer of piezoelectric quartz crystal will vibrate at a specific frequency and the electric current generated can be amplified and fed back to form a harmonic system. Just like old grandfather clocks with pendulums except that the mechanics have been replaced with electronics.
The caesium clocks work on a completely different principle. It is based on the way that materials glow at specific frequencies when exited. You may have seen red neon lights or blue hydrogen lights. For measuring time caesium-133 have been selected as the definition of time, its output frequency is the definition of time. Unlike piezoelectric crystals an atom requires that you interface with it using light, so you can not just hook up wires to it but need a lamp to excite it and a light sensor to read its output. You can use other atoms then caesium and this might be cheaper but caesium gives the most stable results. Hence why it is used for the definition.
Quartz is a crystalline structure that resonates at a precise constant rate – so a cheap clock (oscillator) can be made using quartz as the “internal metronome” with precision on the order of 10’s of microseconds.
Cesium (used in standards) is a radioactive element that undergoes nuclear decay between states at a very very precise constant rate and an expensive oscillator can be made on sub-nanosecond precision (Wikipedia says it’s at 9.19 GHz, so 100ps)
There’s some excellent answers here on caesium already so I’ll focus on quartz.
Quartz is a compound, rather than an element — it’s *silicon dioxide*. Like many crystals it is naturally *piezoelectric* — it can convert mechanical stress into electric current, and vice versa. This means it can be used as a sort of “electronic pendulum” to deliver precisely timed pulses of electricity when connected to a tiny amplifier circuit. There are many types of piezoelectric crystals, but quartz hits the sweet spot in many areas:
* It’s incredibly cheap because it’s literally *everywhere*. Sand is mostly quartz. It accounts for about 12% of the makeup of Earth’s crust.
* It’s really easy to grow artificial quartz crystals of specific sizes and vibrational frequency on an industrial scale.
* It’s “good enough” for a huge number of uses because even the most basic silicon oscillator is ~100 times more accurate than the best mechanical timepieces — the ones in digital watches are usually tuned to 32,768Hz, for example.
* Quartz’s properties don’t change much over human-normal temperature ranges; a quartz oscillator in the depths of winter will work about as well as one at the height of summer, at least as far as human perception is concerned.
*^(Edited for spelling.)*
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