Black lines on the surface of the teeth can be caused by dental caries, hyperpigmentation, dental fluorosis, etc. 1. Dental caries: This is due to cavities in the teeth, which mainly occur in the fossa position of the teeth and can lead to look like black lines on the face of the teeth. Dental caries will usually be irritated by cold, hot, sour and sweet sensations. 2. Hyperpigmentation: It may be due to exogenous hyperpigmentation caused by tea stains or tobacco stains, which leads to the appearance of black lines on the surface of the teeth. 3. Dental fluorosis: This is caused by living in areas with high fluoride content for a long period of time before the age of 7, thus affecting the permanent teeth and resulting in dental fluorosis. High fluoride can inhibit the vitality of alkaline phosphatase, leading to abnormal development of tooth enamel and the appearance of black lines. If the symptoms of black lines on the surface of the teeth continue unabated, affecting aesthetics or causing discomfort, prompt medical attention is required.