Crystals above 30 MHz (around >200 MHz) are typically operated at sequence resonance in which the impedance appears at its minimal and equal towards the series resistance. For these crystals the series resistance is specified ( Predecessor of surface area mount technology but nevertheless beneficial in certain eventualities, including prototyping https://thesocialroi.com/story8770607/the-2-minute-rule-for-smd-crystal-oscillators