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.

Adularescence in rainbow moonstone

Adularescence: A milky, bluish-white glow caused by scattering of light related to the crystal growth pattern.
Gems: moonstone

Asterism (star gems): Crossing bands of reflected light in the gemstone create a four- or six-rayed star in the dome of a cabochon.
Gems: ruby, sapphire, garnet, spinel, diopside, beryl, quartz, moonstone, sunstone

Aventurescence: A glittery effect caused by light reflecting from small, flat inclusions within a gemstone.
Gems: sunstone, aventurine, sheen obsidian

Tiger’s eye

Chatoyancy (cat’s eye gems): A display of a band of light, like a cat’s eye. It is caused by the reflection of light from many parallel, needle-like inclusions inside a gem.
Gems: garnet, ruby, sapphire, chrysoberyl, tourmaline, beryl, tiger’s eye, quartz, moonstone, obsidian, and others.

Color-Change: Gem changes color when exposed to different types of light.
Gems: sapphire, garnet, alexandrite, spinel

Next month I’ll cover iridescence, labradorescence, orient and play of color in part 2 of this blog post.

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