What causes iridescence in gemstones?

Iridescence in gemstones is primarily caused by a phenomenon known as diffraction, which occurs when light interacts with the thin layers of the material within the gemstone. When light passes through these layers, it is broken up into its spectral hues, producing a rainbow effect. This optical effect is often observed in gemstones that have an internal structure or surface features, such as thin layers, that can diffract light.

For example, stones like opal exhibit iridescence due to the presence of a regular arrangement of silica spheres that create an intricate play of light. Similarly, the layers in certain types of pearls or layered agates can also create iridescent appearances. This characteristic makes gemstones with iridescence visually striking and sought after in jewelry.

In contrast, flaws within a stone may affect its overall clarity and brilliance but do not specifically create iridescence. A natural chemical reaction may lead to changes in color or surface texture but does not account for the interplay of light that defines iridescence. The gemstone's overall color can influence its appearance but is not a direct cause of iridescence itself. Thus, the correct response encapsulates the essence of how iridescence arises through the interaction of light with layered structures in the gem's composition.