For a crystal oscillator to work, there must exist a feedback path with properly designed specifications in order for all to work as expected. Broadly, the basic form of this control system is as follows:
Where the transfer function of this control system is:
$$\frac{Vout(s)}{Vin(s)} = T(s) = \frac{G(s)}{1+k(s)G(s)}$$
The theory of this type of oscillating circuit is such that if we make
$$k(s)G(s) = -1$$
Then we will have
$$T(s) = \frac{G(s)}{0}=\infty $$
And with zero input to the system (i.e. self-contained feedback only)
$$Vout(s) = 0*\infty$$
Which is undefined and together with the -1 feedback we find the system to be unstable and oscillatory in nature. What practical circuits do is design such that we have a phase-shift of 180 degrees and a gain of 1 equating to the -1 requirement. Namely, the feedback path k provides the 180 degrees phase shift and the forward path, G, provide the unity gain.
All of this together (with the crystal oscillator and phase-shifting load capacitance) give a circuit that is unstable and will oscillate at the resonant frequency of the used crystal.
tl;dr
Now, the point of all of that was to setup for a simple question. Knowing that the gain in the forward path must be unity, it makes sense that we would use something like an op-amp configured as a voltage follower, or equivalent. Since the oscillator will oscillate in a sinusoidal manner, it makes sense that we would use an amplification method that would track the input. However, what I've found is that these circuits tend not to use op-amps but rather high bandwidth CMOS inverters (like CD4049). Of the type:
How could it be that we are able to use an inverter to provide this unity gain with no additional phase shift?
The datasheet for the CD4049 cites that low voltages is anything below 1V, while high voltages are defined as being greater than 4V. That said, how can this be considered sufficient as a unity-gain amplifier when the output is either HIGH or LOW and not actually tracking the input as one would expect with something like a voltage follower?
I have actually constructed and measured this circuit; here are the results.