(from Trivial Pursuits, Albuquerque Gem and Mineral Club)
One of the more striking types of color mechanisms is the one causing a play-of-colors. This is what gives those beautiful iridescent colors to gem minerals like labradorite, peristerite, moonstone, opal and fire agate. It is sometimes seen on tarnished sulfides like chalcopyrite or pyrite and occasionally on an oxide or a silicate mineral.
Is this mechanism caused by electrons like most color mechanisms? No, it is caused by an optical effect called interference. When light strikes a shiny, transparent layer at an oblique angle the light rays are split - part of the light is reflected off the upper surface and part goes into the layer. Some of the light that enters the layer is reflected off the bottom surface and exists parallel to the other reflection. However, white light is composed of a numbers of colors - each of which travels at a different speed in the layer. So when the split light rays emerge from the layer they have been retarded and their wave patterns are out of synchronization with the part of the light reflected off the top of the layer. Or rather, MOST of the rays are out of synch. One or two colors will be in phase and the rest out of phase. The in phase color will be very bright and the out of phase colors will be very pale or absent. Thus, we may see a rich, glowing color coming from a colorless, transparent stone, but only in one special direction.
The intensity of the color and the hue are both affected by the thickness of the layer. If the layer is very thin, about the thickness of one wavelength of blue light, only blue will come out. If it is just a tad larger, the color will be green. As the thickness increases, the colors change from intense blue to green to yellow to red to medium blue-green-yellow-red to pale blue-green-yellow-red and so on. (yep-only four colors) You can see this to advantage on any oil slick. If you put a drop of oil on water, it will form sets of iridescent rings on the water with the brightest on the outside edge and a sort of milky haze in the center.
What can cause these kinds of layers? Several things like thin cracks (calcite, opal), thin layers of a different mineral inside a host mineral (peristerite, moonstone), thin layers of two polymorphs of the same mineral (labradorite), and thin surface coating (tarnish on sulfides, etc.). Labradorites have sharp crisp colors because their layers are usually thin while moonstones have pale blues or even gray because their layers are very thick (comparatively).
The best precious opal has another facet to this story but I’d better catch that some other time.
Republished by permission of Albuquerque Gem and Mineral Club
Paul Hlava, author