Here’s a very cool Hackaday.com post on the development of radio-crystal manufacturing during the Second World War. Xtals had been used experimentally before the war, but the need for reliable communication transformed amateur radio based mostly on coil and capacitor oscillators to the widespread use of much-more stable crystal ones.
http://hackaday.com/2015/03/03/retrotechtacular-crystals-go-to-war/#more-147734
