For transmitters and receivers that use crystals, they are made to work in a specific type of resonant circuit. A crystal will oscillate at a fraction of the intended frequency, and the circuitry then resonates at the correct frequency. There are also other electrical considerations, such as capacitance, inductance, and the like. That's why different brands of systems use different types of crystals. Also, receiver and transmitter crystals are different, oscillate on different frequencies, so you can't interchange them. Remember, the circuitry plus the crystal are needed to work on the proper frequency.

It's a more complex subject that really takes a chapter of an electronics textbook to go over. The above is extremely simplified.