Asterism in Gems
by Nasrina Parvin Khan
“Asterism” is an exclusive light effect (which we called special optical phenomenon) of some crystal. When light pass through these crystals, its exhibit a star-shaped figure, which called “asterism”.
Asterism” is a term, which is most commonly used in gemology. It refers a kind of special optic phenomenon that has been seen in several gems.
Special optic phenomena refer uncommon and excellent optic properties exhibit by some rare gemstone. The phenomena are particular property of transmitting or reflecting light. The structure or inclusions of a mineral in gems is the reason of this property. The beauty and value of gems depend on their optical properties.
“Asterism”-one of the important optical phenomena 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 guess “asterism” is star-shaped light effect across the surface of gems.
“Asterism” is an exclusive light effect (which we called special optical phenomenon) of some crystal. When light pass through these crystals, its exhibit a star-shaped figure, which called “asterism”. This figure is produced by reflected or transmitted light.
Asterism in Gems
Gems, which show an illuminated star, just bellow the surface of crystal called star gems. The most popular star gems are star ruby and star sapphire. Other gems, which also possess the special optic phenomenon “asterism”, are rose quartz, garnet, spinel, Chrysoberyl and Diopside. There are a few specimen of rock crystal, which has the phenomenon.
The star in the star gems may have four to twelve rays depending on the crystal system of the gems. But the six- rayed star effect is most common. Usually, six-rayed star show by the star rubies and star sapphires. Sometimes these star gems exhibit the twelve-rayed star effect. But the twelve-rayed star effect in corundum is occasional. Rose quartz exhibits six-rayed star. Some garnets and spinel commonly have the star effect with four rays, but these star gems can show six-rayed star effect if cut in a distinct way. Some crystals like Chrysoberyl and Diopside only exhibit four-rayed star effect.
“Asterism” is caused by the dense inclusions of tiny fibres of foreign minerals in gems. Generally, these inclusions are the reason of the reflection of light and form a wavy star like formation, which moves around when the mineral is rotated. All the inclusions in the gems are formed via exsolution.
The inclusions must be long, very thin needle like which called as rutile. “Asterism” is produced when needles intersect each other in multiple groups. Small needles of rutile (titanium oxide), sometimes called “silk” are oriented inside the minerals in the direction of the crystal growth system. There is a relationship between the gem’s growth and the number of rays produced by the reflected light.
There must be three or more chatoyant bands inside the gems to produce “asterism”. The arrangement of needles in a hexagonal pattern produced six-rayed “asterism”.
Exsolved inclusions form a pattern in the host mineral. The pattern may vary from mineral to mineral, but in case of specific mineral the pattern remain same. In case of corundum, we can see rutile is exsolved in three directions crossing at 60 degree or 120 degree in the basal and produce six or twelve-rayed star. When the rutiles crossed at 90 degree, it produces a four-rayed star. Star gems are translucent to opaque .The rays of star should cross at the top of these gems and be distinct.
The sharpness and the size of “asterism” in gems, which identified as phenomenon, depend on the quality and quantity of rutile needles. Small to smaller size of rutile needles help “asterism” to be sharper. When the needles are visible with open eyes as inclusions, the gems turned into a low translucent stone and lack “asterism”.
Large quantities of rutile needles in a crystal produce phenomenal “aster”. If the quantities of rutiles are small, “Asterism” is visible in only a small area. These gems are not recognized as phenomenal gems .The clarity of these gems can be improve by dissolving some rutile needles with heat treatment. Heat treatment allows various elements to be incorporated into crystal. Then rutile needles form by very gradual cooling system and induce “asterism”.
Inclusions are not present in all star gems. Some specimens leave holes or empty areas in the host. Reflection creates from fibres or fibrous cavity in the crystal.
Types of asterism
There are two types of “asterism”, like Epiasterism and Diasterism. Epiasterism is creates by the reflection of light on parallel-arranged inclusions inside the crystal. Diasterism is creates by the transmitting light through the stone, mostly seen in rose quartz. To see this effect a lighted background is essential.
To display the star effect in best way a gem is cut as cabochon with the girdle parallel to the needles. The domed stones are round or oval shaped.
The special optical phenomenon “asterism” is most visible in direct, single pointed light source, like an incandescent bulb, fibre optic light, and penlight or in sunlight.
The distinctiveness of the star and degree of strait ness of each ray help to judge the quality and value of star gems.