> This is actually an interesting question.
> Does it have to be a sine wave like a xtal's? Or will a square wave
> work?

Works fine.  Check the spec though, for levels, rise and fall times, and
duty-cycle.
Old trick on the Z-80 was to run it on an asymmetrical clock, which let you
speed it up quite a bit beyond it's max with a 50% duty cycle clock.