The darker zone seems to be a separation zone to make three terminal piezo transducers possible. They are used in sounders. I guess the outer ring and the larger grey area are the wanted terminals.
Here is a related story: What's the third wire on a piezo buzzer?
If you have a sensor in your hands, take a multimeter which can measure voltages well below 1V. Find those terminals which have an output when the sensor is knocked. Solder the wires to them. One of the terminals can (but that's not any universal truth) be the metal case if there's one.
Use thin multistrand wire. Make sure the solderings do not get stretched or twisted via the wiring.
You probably don't want harvest AC hum nor noise, so use shielded mic cable. Learn something about mic wirings. I guess your cables are short, maybe only 2 meters. Unbalanced wiring can work well in that case.