Interference

Some gemstones, such as moonstone, are "pseudochromatic." That is, of growth has caused the development of various kinds of films or layers. Interference colors are the usual result. In describing the process of interference, the usual procedure is to invoke an image of the rainbow play of colors on a thin oil slick on a rain-wet street.

A ray of light strikes the thin layer of oil at some angle. Some of the ray is reflected immediately from the top surface of the oil; some penetrates the thin film, and, in turn, is reflected from the contact surface where the oil film rests on the water. This second ray portion, traveling back through the oil film, continues on its way parallel to the first ray fraction.

However, it is retarded because it has traveled a slightly longer distance. This means that the light waves in the two parts bouncing back have gotten out of step with each other. Since light waves are additive, the resulting combination of out-of-step portions in the eye of the observer is of a different mixture of wavelengths from the original ray or, by definition, a different color blend. The hue produced by these interfering wavelengths depends on the thickness of the film and the angle at which the ray of light strikes it. If the film is too thick or too thin, interference effects are lost. Moonstone offers a good example of the "schiller," or glow of color produced by interference effects. It contains very thin layers of the mineral albite alternating with very thin layers of the mineral orthoclase. These layers act as films, thus producing the popular bluish and ghostly internal glow by interference when struck by a ray of light. The play of iridescence or tarnish colors on some metals is interference color due to the formation of very thin films of various oxides or sulfides left on the metals by the chemical attack of gases or solutions.