I just spent some time understanding the shape of the quartz crystal (don't be fooled by the broken disk in the picture -- it was originally a circular disk.)

I just spent some time understanding the shape of the quartz crystal (don't be fooled by the broken disk in the picture -- it was originally a circular disk.) I expected a tuning fork shape. Which indeed are used for low-frequency crystals like 32 kHz. This one is a disk that vibrates in shear mode. The crystal is cut on an angle (about 35 degrees) to the crystal grain structure. When voltage is applied across the thickness of the disk, it creates a force parallel to the crystal axis. The crystal is extremely stiff in compression, so it can't get thinner, but it can shear so the top moves one way and the bottom moves the other. The reason behind this complicated setup is that it's the most stable over temperature. The stiffness of any material changes with temperature, but when you get the angles exactly right the changes cancel out. Further reading: https://en.wikipedia.org/wiki/Crystal_oscillator#Crystal_structures_and_materials https://en.wikipedia.org/wiki/Crystal_oscillator#Crystal_str... https://www.jauch.com/blog/en/its-all-about-the-angle-the-at-cut-for-quartz-crystals/ https://www.jauch.com/blog/en/its-all-about-the-angle-the-at...