Iris Quartz

Iridescent cracks form naturally when thin films of air are trapped in fractures, and GIA researchers note that iris quartz crystals occur worldwide. A well-studied type comes from geodes in the Deccan Trap basalts of west-central India, including the Jalgaon District; there, Brazil-twinned quartz was later etched by fluids along its twin boundaries, creating a surface grating that splits light into colors.

In fracture-type iris quartz the colors appear along internal crack planes, while in metal-coated quartz the iridescence lies on the outer surface. Heating and rapid cooling (quench crackling) can create iridescent cracks artificially, and the International Gem Society lists this treatment of quartz as undetectable, so natural origin cannot always be confirmed. In the Indian type, GIA found iridescence only on the minor z faces and not on the major r faces.

In Indian iris quartz the effect lies in a thin surface layer: when GIA researchers polished iridescent faces, the iridescence first intensified and then disappeared. Such crystals should therefore not be repolished or abraded.

A specimen of iridescent Indian quartz was donated to the British Museum (Natural History) in 1860, and the crystallographer Gerhard vom Rath described its play of colors in the early 1870s. Mindat reports that from 2006 tons of such quartz were collected in India for a Japanese buyer, who released it in 2009 at Japanese mineral shows as "rainbow quartz"; because that name was already used for other materials, Mindat authors adopted "iris quartz", by analogy with iris agate.