Black gums can be caused by hyperpigmentation, chronic hypoadrenocorticism, dental calculus and so on. 1. Hyperpigmentation: pigment is deposited in the gum area when the gums are developing, and some people will have a dark circle of gums. 2. Chronic Hypoadrenocorticism: Chronic Hypoadrenocorticism may be caused by autoimmune disorders, infections, tumors and other factors. Patients will have blue-black pigmentation of the gums, as well as other manifestations such as weight loss and decreased blood pressure. Therefore, patients with chronic hypoadrenocorticism will have the symptom of a circle of black gums. 3. Dental calculus: dental calculus is caused by plaque infection, mineral deposition and other reasons. Yellowish-white or brownish-black calculus will appear at the junction of neck and gums of the patient, thus the patient’s gums will have a black circle. 4. Other: patients with a circle of black gums may also be metal poisoning, plaque gingivitis and other causes. It is recommended that patients with black gums need to consult a doctor in a timely manner, the doctor will choose the appropriate measures according to the cause of the treatment.