Iridescent glass has been flash-fired with a micro-thin layer of metallic crystal, creating jewel-like colors in reflected light.
Louis Comfort Tiffany started making a type of iridescent glass back in 1896. It was called Favrile glass and is different from the iridescent glass we see today because the Favrile glass had the color ingrained in the glass itself. 
We now have a glass called Luminescent. It is a low-fire glass coating on fusible glass. The coating's appearance changes depending on how it is fired and whether the coated side is up or down. Best if fired face down so it does not burn off. Fire no higher than 1400 F.
