Iris quartz, sometimes also marketed under the name "rainbow quartz" or "anandalite", is a quartz crystal displaying internal spectral colours (single colours or tiny rainbows) under some of the crystal faces (normally the minor rhombohedral faces). These internal colours are interference phenomenon due to reflection and refraction on extremely thin parallel brazil-law twinning lamellae. The quartz crystals themselves are generally colourless or faintly amethystine or citrine, and almost always from vugs in basalt or other trap rock. Such crystals are occasionally facetted, with the rainbow colours displayed as an internal optical phenomenon.
Not to be confused with quartz crystals that show rainbow colours on the surface, rather than internally, because of thin films coating the exterior of the crystal. These are sometimes also sold under the name "rainbow quartz" but obviously can not be cut as gems because the colourful surface film would be lost.
