The real black line of porcelain crown is actually gray staining, which is a layer of misty gingival staining. This is the real gum "black line". It is a symptom that metal ions are deposited on the gums after precipitation. But it is not harmful to health, but only slightly affects the appearance.

When you can see the black line, it means that the porcelain crown is not in close contact with your teeth, which will cause food accumulation, erode your teeth and make the porcelain crown fall off early.