http://www.crystal-radio.eu/set9/enset9.htm

"a resonator is essentially a low-budget crystal" from:
http://electronics.stackexchange.com/questions/36308/crystals-oscillators-and-resonators-what-the-difference

Drifting off frequency might not be good for a radio? (Or it might be good? see last link.)
"Prior to crystals, radio stations controlled their frequency with tuned circuits, which could easily drift off ... "
https://en.wikipedia.org/wiki/Crystal_oscillator

- Brian

"This wide swing is useful for some radio transmitter and receiver circuits. However in other cases the accuracy and repeatability of a quartz crystal oscillator may be preferred."
http://www.eevblog.com/forum/testgear/can-a-ceramic-resonator-be-substituted-for-a-quartz-crystal/

There are some interesting coil winding websites.