The term 'iridescence' refers to lustrous waves of color can be detected in some gemstones. Other terms for iridescence include 'fire' and 'play of color'. This rainbow-like effect is the result of cracks or layers within the stone that change the way it refracts light when it is viewed from different angles. An 'iridescent finish' is an artificial form of this; the treatment of applying a rainbow-metallic finish is also referred to as 'iris' or 'AB finish'. The 'AB' here is an abbreviation for 'aurora borealis' -- a rainbow finish that can be applied to virtually any color or type of bead. The aurora borealis, also called the northern lights, is the phenomenon of shimmering colored lights that appear in some locales -- a most incredible sight to behold. In beads, this 'shimmer-effect' is created by adding a very thin coat of metal. One stone with natural irridescence (in my view, the most precious and what Shannon and I seek in gemstones) is the fire agate, shown here as featured on Etsy.com by '5andUnder'.