Is it a Diode, or Zener or what

[boater sam]

First thought. Connect to variable power supply via a resistor to limit the current. Ammeter in series, voltmeter across. Wind up the voltage until it conducts. Note the readings. Then try it with the DIAC reversed. Should give same result. You may need 40V or so to make it conduct.

Unless you have the luxury of a curve tracer. Then you can see the whole characteristic on the screen.

Thanks, that's tomorrows job.