Which type of stain cannot be removed by polishing?

Intrinsic stains are those that occur within the structure of the tooth, making them more challenging to remove compared to other types of stains. They are typically associated with factors such as developmental anomalies, certain medications (like tetracycline), excessive fluoride exposure, or even trauma to the teeth. These stains are embedded in the dentin and enamel, which means that superficial techniques like polishing, typically used to address extrinsic stains, will not affect them.

In contrast, extrinsic stains, which are often due to external factors like coffee, tea, or tobacco use, can generally be removed through polishing and proper oral hygiene. Stains from food and surface stains are also considered extrinsic and can typically be dealt with through professional cleaning procedures or at-home dental care routines.

Thus, the reason intrinsic stains cannot be removed by polishing lies in their location within the tooth's structure, necessitating professional intervention such as whitening treatments or restorative procedures if removal or cosmetic correction is desired.