Crystals are required in many applications, which means you’ll often be faced with a decision between using a fundamental or a 3rd overtone device for your necessary frequency. Fundamental resonant frequency is inversely proportional to the crystal’s thickness, which can cause issues at higher frequencies. Simply stated, the crystal will have a risk of breaking when running in a higher frequency. That’s why at higher frequencies, most crystals will be designed to operate on the crystal’s 3rd overtone.
Referring to the above figure, 3rd overtone crystal resonance is 3 times its fundamental frequency. You may also find that there are some spurious responses which are by-products of other vibration states.
A quartz crystal has an equivalent circuit of a series RLC for the fundamental and each harmonic mode, plus a capacitor in parallel representing the capacitance of the electrodes. The resistance of a 3rd overtone crystal is more than 3 times that of the fundamental once and its capacitance is almost 9 times. This affects its Q parameter and crystal tunability. 3rd overtone crystals have better stability performance (higher Q), but are much more difficult to tune. So it behooves you to avoid using 3rd overtone in oscillators with compensation circuitry (VCXO/TCXO/OCXO) if possible.