Iris agate is another, lesser-known kind of iridescent agate. Iris agate is also called rainbow agate, although some vendors simply call any agate that is colorful, "rainbow agate," even when it does not have the iridescent effect. When true iris agate is thinly sliced, it shows all the colors of the rainbow! The effect is more delicate than fire agate. This rainbow effect is the result of very tightly packed, very thin layers and banding, and can occur in colorless or pale chalcedony. It is also mostly found in North America, although some has been recorded in Argentina. Because it must be sliced so thinly, iris agate is fairly rare in jewelry.
Iris agate slice - image courtesy of Karen Brzys, Gitche Gumee Museum, Grand Marais, MI www.agatelady.com and www.agatelady.blogspot.com, used with permission.