Phenomenon:(noun) A special (optical) effect displayed by a gemstone when it interacts with light. Gems with beautiful optical effects are often more valuable.
Here’s a short description of types of phenomena recognized in gemology and the gems commonly associated with each. A gem can have more than one phenomenon.
Iridescence: A rainbow display that is created when light is broken up into different colors like in a soap bubble or oil slick.
Gems: iris agate, fire agate
Labradorescence: A broad flash of color caused by light interacting with a type of crystal growth called twinning.
Orient: A shimmer of iridescent rainbow colors on or just below a pearl’s surface.
Gems: pearl, mother-of-pearl
Play-of-Color: precious opal is made up of sub-microscopic spheres stacked in a grid-like pattern. As the light waves travel between the spheres, the waves diffract, or bend and break up into the colors of the rainbow, called spectral colors.
Gems: opal, fire opal
Last month I covered adularescence, asterism, aventurescence, chatoyancy and color change in part 1 of this blog.
Gemological Institute of America, Gem Identification Lab Manual, 5/2012