[Diabolical Artificer]

"Some of the signal diodes with low breakdown voltages can be rather valuable. Tunnel diodes, microwave noise sources and microwave detector diodes." I have some diacs in the package Marc mentioned too, no idea if they come under that catagory.

Diode type's are often indicated on the PCB by symbol usually hiding under the diode, or by the prefix ZD or similar which you probably know. The very rarely go wrong but on some circuits I've had to replace several.

To check mine I use a little zener checker someone off the forum made and let me have, have the plan's if you want them or you can check them with a V/I component checker that uses your XY facility ont scope. Either way it entails lifting their legs, bit like sexing dogs or newts, but like birds or
spiders, diodes are hard to sex too : ) Easiest ID as Alistair says is if they're upside down.

Andy.