This is great explanation of how the piezoelectric effect of quartz is used to keep accurate time. via Reddit

The amazing everyday wristwatch: We never think about it, but only because engineers have made it so reliable and durable that we don’t need to. At its heart lies a tiny tuning fork made of the mineral quartz. In this video Bill takes apart a cheap watch and shows extreme close-ups of the actually tunings fork. He explains how the piezoelectric effect of quartz lies at the heart of the watch’s
operation.