"Lobster" wrote in message
...

I've noticed two different types of these; some are apparently made of
brass and have a completely rounded external profile, whereas others
seem to be copper, and have flat chamfers at either end.

Can anyone tell me the significance of the two species, and are they
interchangeable in different compression fittings?

Copper is best used where the compression fitting is used as a disconnection
device at an appliance. The brass olives can indent the pipe, copper rarely
do this. On each disconnection and re-connection a new copper olive is
used.