The current porcelain teeth are mainly two categories: the first major category is the inner layer of metal, covered with ceramic metal – porcelain crowns (generally known as porcelain teeth); the second major category is the inner layer, outside the use of ceramic material all-porcelain crowns (generally known as all-porcelain teeth). For porcelain teeth, the cervical gums darken for the following reasons: 1 because the inner layer of metal can not transmit light, so the external light can not be transmitted to the root of the tooth like natural teeth, the cervical gums darken and darken. This kind of darkening can be seen when the porcelain teeth are worn, and it is often difficult to overcome this phenomenon with metal porcelain teeth; 2 internal metal oxidation, metal ions precipitation, deposited in the cervical area, resulting in darkening of the gums. This kind of darkening appears in the porcelain teeth after wearing a period of time, the choice of better metal material is not easy to oxidation will greatly reduce the risk; 3 due to porcelain teeth production is not refined and accurate, porcelain teeth neck and teeth are not close, stimulating the gums, resulting in gum inflammation, inflammation of the gums are red, dark, look black. This darkening also occurs after a period of porcelain teeth wear, if you choose a regular hospital, regular doctors, standardized operation, strict operation can often be avoided. For all-porcelain teeth, because all-porcelain can often transmit part of the light, there is no metal ion precipitation, so as long as the operation is rigorous, careful, the use of all-porcelain resin bonding material, generally will not appear gum darkening phenomenon. This is one of the important reasons why all-porcelain restorations are now becoming more and more popular among patients.