Calculus is a hard, pale, brittle substance formed on the tooth by precipitation of calcium phosphate from saliva onto plaque.

 

Calculus is tenaciously attached to the tooth surface and is not removed by ordinary brushing but only by the dentist.  Calculus above the gum margin (supra-gingival calculus) is very common and not normally a problem

The rarer form, sub-gingival calculus, harbors plaque under the gum margin, preventing it from being brushed off.  This can allow periodontitis to set in.