Coated Rainbow Quartz

This material is produced, not found in nature. Quartz pieces are placed in a vacuum chamber, and metal is evaporated so that its atoms travel through the chamber and deposit as a film on the quartz; the film can be made up to a micron or two thick, and several metals can be deposited together. The rainbow colors are interference colors produced by the thin surface film, the same physical effect seen in an oil film on water.

The iridescence is an interference effect on the outer surface. GIA notes that once a coating is suspected it is easy for a skilled gemologist to identify, and in gold-coated "Aqua Aura" quartz it observed unnatural coloration at facet junctions and an irregular color distribution on some facets. Natural iris quartz differs because its colors come from thin films of air in internal cracks, and in quench-crackled quartz they come from induced cracks, rather than from a coating on the surface.

The colors exist only in the surface film. GIA notes that thin-film coatings of any kind can scratch, especially along edges and facet junctions, and that coated quartz needs care during jewelry manufacture or repair. Keep coated pieces away from hard or abrasive objects and store them wrapped in soft material in a dry place.

Metal-coated quartz entered the gem trade in the late 1980s with gold-coated "Aqua Aura". GIA reports that in the 1990s new colors and effects were created in coated quartz with different combinations of metals, including a "rainbow" iridescence reportedly caused by a silver/platinum coating (Koivula and Kammerling, 1990). Mindat records that quartz coated with titanium or niobium films has been marketed as "rainbow quartz", a name that has also been applied to several natural iridescent quartzes.