Phenomena gems are gems that display unusual optical effects. These optical effects can be brought about by a concentrated beam of light and different types of lighting conditions when light interact with the inclusions and the particular inner structure of the gems. Phenomena gems are often cut as cabochons and tablets to bring out the full impact of the phenomena.
When you are viewing any gemstones use a strong concentrated light source, such as a fiber-optic light or a powerful penlight and move your light above the stone to look for the phenomena. Learn some fundamentals and you will have a greater appreciation of what nature has endowed mankind with its various gemstones from the earth. (Read here for Fundamentals)
The more define the phenomena effect the more valuable and beautiful will be the gemstone. And that include the price too. For example, a well formed centrally placed six-ray star in a ruby cabochon will be very valuable as compared to a weakly formed star that is off center. Or for a color-change Alexandrite, the more vivid the color change under different lighting conditions the more valuable will be the gems.
There are 9 types of Phenomena that occur in various gemstones as follows. The gemological term for the phenomena is described in the first word/s:
Asterism – A Six-Star that forms at the apex of the dome of a cabochon in rubies or sapphires. As your light source moves the star will follow suit. Gemstones that have this phenomena includes Corundum (Read here) and Quartz.
Chatoyancy – The Cat’s Eye effect on the dome of a cabochon with a band of bright light across the gemstone, usually it is vertical on the broadest side, like an eye. As your light source moves across the dome the ‘eye’ will follow. Cat’s eye gemstones include Chrysoberyl and tiger-eye Quartz.
Color-Change – A gem with Color-Change phenomena is one which changes color according to the type of light source. For example, under normal daylight a Color-Change Alexandrite will appear green while under incandescent light the same Alexandrite will appear red.
Play-of-Color – The flashes of rainbow color in an Opal is called Play-of-Color. For black body Opal this Play-of-Color produces a beautiful spectral color across the surface of the stone as it contrast with the black base.
Iridescence – This is the presence of spectral color on the inside or outside of a gemstone. Gems that show iridescence include ammonite, fire agate and iris agate.
Orient – This is a form of iridescence found on the nacre of pearls. The orient is usually cream, silver or black.
Adularescence – This is a billowy light across a Moonstone Feldspar which is normally light blue and shimmering.
Labradorescence – This is a broad color flash of rainbow color across Labradorite.
Aventurescence – This is the color spangling of Sunstone or the glittering light from green Aventurine Quartz.
The cause of each phenomena will be explained in forth coming individual postings.
Photo courtesy from the following websites:
http://www.gemselect.com, http://www.mineralminers.com, http://gemstonepedia.wordpress.com, http://blog.rubylane.com/node/1051, http://www.flickr.com/photos, http://www.tigersoftware.com, http://runakodesigns.blogspot.com, http://www.ommanihealing.com, http://www.gehnabazaar.com, http://multicms.rdts.de/, http://www.bridalpartytees.com, http://www.rawpearls.com.au, http://www.trinketsbythesea.com/, http://www.sinkankassymposium.net, http://geology.com, https://www.juniorgeo.co.uk, http://polymath07.blogspot.com, http://www.palagems.com, http://www.esva.net/, http://www.yourgemologist.com, http://www.squidoo.com, http://www.relichunters.ca