Unlike other gemstones, pearls are not minerals but the product of mollusks. When a foreign object enters a mollusk’s shell, the mollusk will attempt to cover the irritant in an iridescent substance much like the interior of its shell – sometimes producing a beautiful lustrous pearl. The color and size of the pearl are dependent on the species of mollusk, while the shape depends on the shape of the foreign object.

Pearls can occur in wild mollusks, but these are very unpredictable and thus quite valuable. More common are cultured pearls, which are started through human intervention. Cultured pearls have a much thinner layer of iridescent coating – called nacre – than their natural counterparts, but can still be quite durable and are an affordable option.

The value of pearls is determined by their luster, surface quality, shape, color, nacre-thickness, and size. In general, round pearls are considered more valuable, as are pearls of rare colors, such as black. Pearl is the birthstone of June.