If a rainbow could be captured and set in a ring then it would be called "opal". Chasing the rainbow (finding the colour layers that exist in the rough rocks) has been an occupation of lapidaries for years.
00000Opal is comprised of tiny spheres of silica. When these spheres are of uniform size and arranged in a three dimensional layer, light is diffracted into the colours of the spectrum and conveyed as a flash of coloured light.
00000Opal is the stone of emotional, physical and mental harmony. Fire opals in particular are supposed to stimulate passion.

Location - Brazil, Mexico.