ASTERISM - STAR IN GEMSTONES
Reflection of light from extremely fine needle - shaped inclusions oriented to the gemstone crystal structure causes asterism.
An asterism is an optical phenomenon displayed by some rubies , sapphires and other gemstones. When light passes through this crystal it exhibhit a star shaped figure on the surface of the gem which is called as Asterism.
The term "Asterism" came from the Greek word "aster" which means Star.
Dense Inclusions of tiny fibres of foreign mineral in gemstones are the reason of the reflection of light and form a star like formation on the surface of the gemstone,which moves around when the gem is rotated.
All the inclusions in the gems are formed via exsolution.
Exsolved inclusions form a pattern in the gemstones.The pattern may vary from mineral to mineral but in case of specific mineral the pattern remain the same.
In case of corundum we can see rutile inclusions is exsolved in three directions crossing at 60 degree or 120 degree in the basal and produce six or twelve rayed star.When rutiles crossed at 90 degrees it produces four rayed star.
Star gems are translucent to opaque mostly.The rays of the star should cross at the top of these gems and be distinct.
To display the star effect in the best way the gem is cut as cabochons with girdle parallel to the needles.The domed gems are round or oval shaped.
The special optical phenomenon "Asterism", is most visible in direct single pointed light source like an incandescent bulb,penlight or "Sunlight ", of course the best source of Single pointed light.