Special Optic Phenomena in Gems

by Nasrina Parvin Khan

Asterism, chatoyancy, play-of-color, opalescence, labradorscene, adularescence and color change are some special optic phenomena which have been seen in gems.

It is said, "jewelry is the best friend of a woman". Yes, jewelry can increase the confidence level of a woman as a beauty wear as well as wealth. Designs of jewelries changed with time but the value remain unchanged.

In the present scenario, gemstone jewelries take a distinct place among the women because of its stunning beauty and the myth "it may brings good luck". The glitz and glitter of a single gemstone can lights up the face and enhance the radiance of a woman's persona. Gems allowed the light to pass through its. This effect enhances its color and brightness and creates an enchanting look.

The beauty of gems justifies with its color, luster, brightness, brilliance, scintillation and dispersion. Optical properties are responsible to create all this qualities of a gem. These are also important to identify gemstones.

But the most important feature of gems is the special optic phenomena. Special optic effects exhibit in some gems is called as phenomena. These uncommon and excellent effects very often add value to gems.

Special optic phenomenon

Special optic phenomenon is a particular property of transmitting or reflecting light. The structure or inclusions of a mineral such as gem is the reason of this property. Asterism, chatoyancy, play-of-color, opalescence, labradorscene, adularescence and color change are some special optic phenomena which have been seen in gems.
Asterism

Asterism is one of most important special optic phenomena, which has been seen in sapphires, rubies, rose quartz and some other gems. The term "Asterism" came from the Greek word "aster", which means star. From the name we can easily guess, "asterism" is star shaped light effect across the surface of a gem. Asterism is caused by dense inclusions of tiny fibers in the mineral. This inclusion is the cause of the reflection of light and forms a wavy star like formation, which moves around when the mineral is rotated. The star may have four or more rays depending on the crystal system of the gems.

Asterism is best seen when a gem is cut as a cabochon. This optic phenomenon is most visible in direct light. There are two types of asterism. Like, Epiasterism which has seen in sapphires and most other gems and Diasterism which has seen in rose quartz.
Chatoyancy

Chatoyancy is another special optic phenomenon. This phenomenon is also known as Cat's Eye effect. Because the reflection exhibits a single bright band of light across the surface of a gemstone and forms a Cat's eye. The eye can be seen at right angles. Inclusions of bundles of parallel particles of fibres of another mineral inside the gems are the reason of silky radiance of gems, which called Chatoyancy. This phenomenon is commonly seen in Chrysoberyl, Tourmaline, and Tigers Eye and very rarely seen in Emerald, which come from Brazil and Colombia.

Chatoyancy or Cat's eye is best esteemed under sunlight or some other point light source. Each of multiple light sources produce a Cat's Eye on the gems and give an unfocused look. In a cloudy day or under fluorescent lights Chatoyancy or Cat's Eye may vanish completely. Usually it is seen on a round shaped stone, which called, as cabochon.
Play-of-Color

One of the common special optic phenomena is Play-of-Color. The iridescent color effects in Opal and several gems described as Play-of-Color. Opal is made of sub-microscopic silica spheres, which is the cause of light diffraction from the surface or the interior of gems. This property helps to produce different colors as the angle of incident light changes. The colors we see depend on the sizes of the spheres.

There are four terms to describe the various kinds of Play-of-Color. A tiny close-set patch of color is called Pinfire, whereas a big, angular close-set patch of is called mosaic. Sweeping reddish bands across the stone is called Flame, whereas blue and green color effect is called as Peacock.
Alexandrite effect

Color change is a distinct optic phenomenon, which has been seen in several gems. Alexandrite is the best example for showing Color change effect. For this reason sometimes it referred as Alexandrite effect. This phenomenon is also found in sapphires and Garnet. Gems distinctly change their color under different lighting source because of its color change effect.
Opalescence, Labradorescence, Adularescence

Opalescence, Labradorescence, Adularescence are some other special optic phenomena which have seen in several gems.

Opalescence is an optic phenomenon, which is commonly seen in opal. The milky aspect of opal described as opalescence.

Labradorescence is an optic effect, which is commonly seen in labradorite. Gems change color because o9f its position with blue reflection for this effect.

Adularescence is most commonly seen in moonstone. This phenomenon creates a milky radiance or wavy light effect in gems. Inclusions of microscopic particles or irregularities of structure of transparent to translucent gems is the cause of blue reflection, which known as adularescence.

It is the fact that special optic phenomena are the reason to create distinct property and attractiveness to a gem and add value.

See pictures of optic phenomena
 
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