Natural pearls and cultured pearls feel slightly rough or gritty when brushed lightly on the front teeth.
Imitation pearls are glassy-smooth, or plastic-smooth, to the bite.
The tooth test does not work as well if the teeth are capped, however, as their sensation is somewhat deadened.
Natural pearls and cultured pearls are both made by irritated mollusks, which coat grains of sand or beads inside their shells with layers of nacre secreted by folds of special tissue.
Nacre is mostly aragonite, a form of calcium carbonate that forms hard lustrous crystals, and these fine crystals are responsible for the grit.
All mollusks can secrete nacre, but oysters do the best job of making pearls.
The best imitation pearls are made by dipping beads in a soup of finely ground fish scales and glue and drying and polishing them.
The surface resembles the luster of pearls but is not gritty.