Patient-friendly definition from the DentistsDirectory.org dental glossary.
Dental calculus — commonly called tartar — is plaque that has mineralised and hardened onto the tooth surface. When the soft bacterial film of plaque is not removed within 24–72 hours, minerals in saliva (primarily calcium and phosphate) begin to crystallise within it, gradually transforming it into a hard, porous deposit firmly attached to the tooth. Once calculus has formed, no amount of brushing or flossing will remove it — only a dental professional with scaling instruments can dislodge it.
Calculus that forms above the gum line (supragingival) is typically yellowish-white and most visible on the back of the lower front teeth and the outer surface of upper molars — areas closest to the salivary gland openings. Subgingival calculus forms below the gum line within periodontal pockets and is typically darker brown or black because it incorporates blood byproducts. Subgingival calculus is more destructive because it harbours the anaerobic bacteria most associated with bone and tissue loss from gum disease.
Calculus itself is not the primary cause of gum disease — the bacteria living in the biofilm on its porous surface are. But calculus provides a rough, ideal surface for bacterial colonisation and its physical presence below the gum line prevents the gum from reattaching to the tooth. Regular professional cleanings remove calculus before it progresses to gum disease. How quickly people accumulate calculus varies significantly — some people build up heavy deposits in three months, others barely at all after six.