When a plate with a gold decorative edge is placed in a microwave oven, sparks appear at the gold surfaces.
Any metal object acts as an antenna for electromagnetic radiation, including microwaves.
Electric currents are set up in the metal, which in turn cause voltage differentials within and between the pieces of metal. Gold rims of plates inevitably become scratched, and so end up with tiny gaps of less than 1 millimetre between sections of continuous gold metal. As a result, when the plate is subjected to microwaves the voltages generated are applied across these gaps. Because air has quite a low breakdown voltage, it doesn't take much to split the molecules in the air and cause sparks. I have to credit Eric Billet of Brunel University for this explanation.