NACRE This is one of the most important aspects of pearl quality because a pearl with thin nacre will not last. (Nacre is the natural substance produced by mollusks to make pearls.) A pearl with thin nacre, while often blemish free, will typically have a chalky or dull appearance unless it has been coated. Coated (or silicon treated) pearls have almost no value, and are often sold without this treatment being disclosed. We do not sell treated pearls.