Iridescent Gemstones

Iridescence is an effect caused by the interference of light at thin films within the gemstone. The gem materials which display iridescence are fire agate, iris or rainbow agate and iris or rainbow quartz. There are many other gem materials which occasionally display iridescence in fractures, but the above materials are best known for this effect.

Fire agate is cut from a special type of chalcedony nodule in which deeper layers are coated with goethite. The goethite layers act as thin films, producing interference and a lovely iridescence. In judging quality of fire agate, color is of prime importance. The most desirable colors are blue, yellow and red, with violet, green and orange being less so. Bronzy or metallic colors are undesirable. Fine stones display fire 90% or more of the face. If the fire covers less than 70% of the face, the body color begins to detract from the stone’s appearance. As a general rule, the less color coverage, the lower the value of the stone. When checking the stone, be sure to rotate it so that any cracks, pits or dead spots can be seen. All of these factors will lower the value.

Iris or rainbow agate is a special type of chalcedony which shows iridescent colors when examined in transmitted light. Agate is composed of multitudes of tiny quartz crystals which build layer upon layer during growth. If enough of these layers are present, iridescent colors will be seen when a thin slice is held against a light. Any agate deposit yielding highly translucent material can produce iris agate, but its presence cannot be determined from the outside appearance alone. The only way to know for sure is to cut a very, very thin slice from the center of the agate nodule, and after wetting it with oil, hold it in front of the light. So that the slice will not have to be tilted to see the rainbow effect, it is important that it is centered properly when being cut. The saw must cut through the bands of the agate at right angles in order to orient the effect properly.

Iris or rainbow quartz is a special type of rock crystal quartz which shows iridescent colors in reflected light. These colors are caused by the interference of light at air-filled fractures throughout the stone. Sometimes the complete range of spectral colors can be seen because each crack starts as a very narrow wedge at one end and gradually gets wider and wider towards the surface. Because of the variable size of the fracture, different colors are seen. The iris effect can be artificially induced by heating quartz and then quenching it (rapidly cooling it) in cold water. If a dye is added to the water, it dries in the cracks, producing what are termed ‘firestones’.