quartz crystal oscillator applied voltage
It's AC, BUT:

It's not really that an AC voltage is applied to the quartz: 

The quartz has the nice property of that if you invert the voltage across it, it takes some time for the quartz to "follow":



For a start, assume the left hand side of the quartz X has just become negative, the right hand side positive. That will lead to the quartz crystal expanding (or contracting) in a specific way, which will overshoot, and a little later invert that voltage. Since that will revert the voltage at the input of the first inverter, that will invert its output – and the cycle starts again.

The specific delay at which this happens due to the mechanical properties of the quartz leads to a resonance frequency that the quartz defines.

So, no, no AC voltage is applied, but with a bit of negative feedback, you can make a quartz oscillate at a stable freqeuncy.