If you own a microwave oven, you've probably heard that you shouldn't put metal in it. This isn't universally true, but it does apply to thin metal plating or crumpled sheets of aluminum foil. Avoid putting any item with gold trim in the microwave unless you can verify that it's microwave-safe.
Thin Metal Overheats
The proscription against metal in the microwave applies mainly to thin sheets or metal plating. Thick metal plates can absorb the radiation generated by the magnetrons in the appliance and can actually be used to shield large food items so they won't cook as quickly. Thin metal sheets, on the other hand, tend to overheat because there isn't anywhere for the heat generated by the magnetic field to go. The resulting spark can damage the inside of the oven and even start a fire.
Cookware, plates and bowls with gold, platinum or silver trim generally aren't microwaveable unless they are specifically designed and manufactured to be put in the microwave. If you aren't sure, it's best to err on the side of safety and leave these items out of the microwave. The metal overheats and sparks, and at the very least, the sparks will probably damage the trim. In the worst case, your oven could be damaged or a fire could start.