Microwaving food on a porcelain plate with a metallic rim can indeed be dangerous and may pose risks. The sparks you observed in the microwave are a clear indication that the plate is not microwave-safe and should not be used for heating food in the microwave.
When a microwave-safe dish is used in the microwave, it allows the microwaves to pass through the material and heat the food evenly. However, materials with metallic elements, such as a metallic rim on a porcelain plate, can cause the microwaves to reflect or generate electrical currents. This can lead to sparking, potentially causing damage to the plate, the microwave, or even starting a fire.
In terms of the impact on the food, the sparks themselves might not harm the food directly. However, the sparking indicates an unsafe condition, and there is a possibility of uneven heating or damage to the plate, which can result in hot spots or other issues. This uneven heating may cause some parts of the food to become excessively hot while other parts remain undercooked or cold.
To ensure the safety of your microwave and your food, it's important to use microwave-safe dishes and follow the manufacturer's instructions. If you have any doubts about the safety of a particular dish, it's best to avoid using it in the microwave to prevent any potential hazards.