When plaque remains on the surfaces of the teeth, it hardens into a darker substance called by dentists “calculus” but which is commonly called “tartar.” It is a light yellow to dark brown in color. It forms on the teeth along the gum line, and has very sharp edges, cutting into the gums as food is chewed. Tartar might be likened to the deposit or scale that accumulates in a teakettle after some use. It forms in layers. Though tartar is mixed with other debris in the mouth, it is, like the scale of a teakettle, made up mainly of calcium.