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 in labradorite

Labradorescence: A broad flash of color caused by light interacting with a type of crystal growth called twinning.
Gems: labradorite

Orient:  A shimmer of iridescent rainbow colors on or just below a pearl’s surface.
Gems: pearl, mother-of-pearl

Koroit boulder opal pendant

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.

Sources:
Gemological Institute of America, Gem Identification Lab Manual, 5/2012
http://4csblog.gia.edu/2014/phenomenal-gems
http://www.gia.edu/opal-description
http://gemologyproject.com/wiki/index.php?title=Sheen