Cavities generally appear black, while normal teeth are white. The structural changes in the dental tissues due to bacterial infection cause the hard tissues of the teeth to demineralize, enlarging the internal spaces of the teeth. This can cause cavities to appear black under the refraction of sunlight, resulting in a black appearance. Additionally, after the deterioration of the tooth due to decay, the breakdown of the hard tissue allows food residues or plaque to enter the cavities, which can also cause the cavities to turn black. This situation requires early dental filling to remove the decayed parts, followed by root canal treatment, otherwise it can lead to pulpitis, causing tooth pain.