Iris Quartz Classification
Colorless, Slightly Purple, Yellow Displaying Internal Spectral Colors
Refractive index, appearance, birefringence, optic character and optic figure. A bull's-eye optic figure is determinative for quartz; may also show Airy's spiral optic figure and Brazil-law twinning under crossed polarizing filters.
This interference phenomenon in iris quartz is due to reflection and refraction on extremely thin parallel brazil-law twinning lamellae. The rainbow quartz name used for other types of quartz, including those showing internal interference colors caused by fractures ("crack rainbows"), and interference colors caused by superficial thin films of limonite or other minerals.