With this simple electronic circuit we can test almost any crystal. The circuit consists of two identical high frequency transistors.
In fact, only the first transistor (TR1) is required to operate well up to 100MHz. In place of the second transistor we may use any common high current amplification low frequency transistor.
The TR1 works as an oscillator. By connecting a functional crystal, the oscillator oscillates at the resonant frequency of the crystal. The oscillator signal is then rectified by the diodes, smoothed by the C4 capacitor and activates the TR2 which acts as a switch and lights up a LED. When the LED lights up, it means that the crystal is functional because it activates the oscillator.
Since the available crystals have resonant frequencies up to 100MHz, there is a requirement that the first transistor has sufficient amplification up to this frequency to be able to act as a resonant oscillator.
If we connect a frequency meter at the output of the oscillator, before the rectification stage, then we will be able to see exactly the tuning frequency of the crystal under test.