When I microwave sliced bell peppers, they spark like they have metal in them. Does the bell peppers have any metal pieces?
Yes. But probably not the way most people think of "metal"
Certain vegetables are very rich in minerals. These minerals, like iron, are an important part of nutrition. It's not metallic in the way we usually think of "metals" being shiny, sharp, and hard.
It's the same iron that in it's pure state gets used to make cast iron pans--but it's in relatively tiny amounts scattered throughout your food and water.
So why do some veggies cause "arcing" in the microwave? Arcing isn't necessarily caused by a lot of metal. It's actually caused by uneven metal content. A flat, smooth sheet of aluminum may not arc at all, but a smaller ball of aluminum foil can cause a major lightning storm in your microwave. Veggies that are particularly mineral rich will be more prone to having uneven distribution between different plants, or even within a single plant.
That means two pieces of bell pepper, either from different peppers or different ends of the same pepper, can have different metal/mineral content, and result in arcing.
Leafy greens in particular have a high iron content, and are prone to this behavior. I've had fresh kale cause some big sparks before. This NPR article goes into more detail.