How is crystal frequency set?
Coarse tuning is done in the grinding process, where the thickness of the quartz wafers is established. I once visited a manufacturing facility; the grinder operators were using ordinary shortwave receivers to monitor the broad peak of electrical noise that the batch of wafers emitted due to the grinding action. When it reached a point slightly higher than the target frequency, they would stop.

Fine tuning is done during the plating process that puts the electrodes on the wafer; adding metal brings the frequency back down. This process is much more precise, and each crystal can be individually calibrated.