This is a specimen of quartz exhibiting iridescence on select crystal faces. Natural iridescent quartz of this kind (also called “iris quartz” and “rainbow quartz”) has been known from the Deccan Traps since the nineteenth century. Analysis of these crystals has determined that the iridescent z faces of the crystals have periodic microscopic surface ridges, creating a diffraction grating, giving rise to the dispersed coloured light. These ridges, resulting from etching, have actually been interpreted as paired Brazil twin boundaries – discussed by Xiayang Lin and Peter Heaney in “Causes of Iridescence in Natural Quartz”, Gems & Gemology, 51:3, Spring 2017 (if you’re interested, it’s online, here). This specimen is in good condition overall, some chipping. There are some brilliant colourful flashes in this specimen if you take some time to examine it! A neat iris quartz from the John S. White quartz collection.