Jewelry Birthstones Wizard
“How to Buy Birthstones Like a Pro” by padre art
Phenomenal Gems
by: padre art
Chatoyance, Asterism and Chroism
The optical phenomena of cats eye and star gems is due to the reflective nature of parallel needle like inclusions, usually of the mineral called rutile, that are too small to see without magnification.
Cats Eye and Star Gems
If there is one set of parallel fibers the optical effect is the single line along the length of the gem termed chatoyance or cats eye. When two or three sets of fibers, that cross each other, are present the gemstone displays a four or six armed star of shimmering light called asterism.
Corundum gems such as star ruby and star sapphire display the beautiful effect of asterism. These asteria, along with the chatoyant chrysoberyl are the next hardest gems after the diamond.
The optical phenomena, chatoyance, is observed most often in the cats eye chrysoberyl, tourmaline and corundum gems. Chatoyance is found less frequently in moonstone and quartz and only rarely in beryl and garnet gems.
Phenomenal Gems
There are a number of different optical phenomena displayed by the feldspar group of minerals. The diffuse glow of the familiar feldspar mineral, moonstone, is called adularescence.
When bright spangles in a gem are caused by tiny separations along cleavage planes the effect goes by the name ‘schiller’ but if the spangles result from small flat platelets of hematite the term is ‘aventurescence’.
Cats eye (called Oculus Solaris or ‘Eye of the Sun’ by the ancients) is not the only optical phenomena of chrysoberyl gems. When chrysoberyl displays a change in color depending on the light source or its orientation the optical phenomena is called dichroism.
Color Changing Gems
These color changing chrysoberyl gems were named Alexandrite after the Czar Alexander who came of age when the gemstone was discovered in Russia. When Alexandrite is viewed in sunlight it is a lovely green color but when seen under incandescent light is a vibrant red.
Other precious stones such as tourmaline, beryl and the corundum gems will show their color changes when the gem is oriented in a different direction to the light source.
To achieve this optical phenomena in these gemstones, as with chatoyance in cats eye, it is necessary for the lapidary to orient the proposed gem in the proper position in the rough material from which it is to be cut.
The best color effect in green tourmaline, for instance, is achieved by orienting the gem across the optic axis of the crystal. The best color in pink tourmaline, on the other hand, is found along the crystal’s axis.
In facet grade iolite and andalusite there is a display of three colors in each gem. This rare optical phenomena is termed trichroism and requires a skilled and experienced lapidary to achieve the correct orientation of the gem in its rough crystal.
Chatoyance, Asterism and Chroism
The already beautiful chrysoberyl, sapphire and ruby gems are made even more intriguing when they exhibit the optical phenomena of chatoyance, an effect that creates cats eye and star gems. The dichroism of color changing gems such as tourmaline adds to the delight of these already precious stones.
- Phenomenal Gems
- Chrysoberyl gem with strong cats eye chatoyancy effect.
- Phenomenal Gems
- Phenomenal Gems
- Star Sapphire gem with clear asterism.
- Phenomenal Gems
- Very large star ruby gem with an excellent example of optical phenomena called asterism.
- Phenomenal Gems
- A beautiful garnet gem with an amazing display of the optical phenomena called dichroism.
Phenomenal Gems
