A Distinctive Glow

One of the most distinctive features of a nacreous pearl is the way it seems to glow from within. This property, known as "luster," gives pearls their unusual beauty. Luster results from the reflection of light rays not only off the surface of the pearl, but also off the concentric inner layers of nacre. Because a pearl's surface is round, it acts as a convex mirror, reflecting light so that it appears to emanate from within the pearl. The multiple layers of nacre also give rise to the "iridescence" or "orient" of pearls--a characteristic that resembles the shimmer seen on a soap bubble. The layers of nacre act like tiny prisms, refracting light so that it appears as all the colors of the rainbow.

Fossil ammonite
Desmoceras latidorsatum
Madagascar, 100 million years old (Late Cretaceous Period)

American Museum of Natural History 46866

Ammonites are extinct mollusks related to the modern squid, octopus and Chambered Nautilus. Under certain conditions related to fossilization, the nacreous shell becomes highly iridescent; the resulting gemlike material is called "ammolite."