Mankind has always been drawn to the mystical skies shinning with uncountable number of stars. Since time immemorial, numerous artworks have been inspired by the stars and planets twinkling. And when someday, gemstones with starry dazzle were discovered, it meant owning the stars. Meanwhile not all gemstones known to man exhibit the starry dazzle and it is an exclusive phenomenon. But why is that? That’s because, it is an optical property based on the crystal structure.
What is the phenomenon called?
The property is called asterism. Asterism is an optical phenomenon that occurs sparingly in some gemstones. When the light beam passes through the crystal, it showcases star-shaped figure. In astronomy, asterism is defined as a pattern of stars. It is very different from what we know as a constellation. Asterism is a pattern of stars or constellations which may not be recognized in any form.
Types of Asterism
There are two distinct types of asterism exhibited by the gemstones found naturally. They are:
Seen in sapphire gemstone and garnet beads, epi-asterism is caused due to the reflection of the light along the parallel planes formed from inclusions/ rutile within the gem crystal.
Commonly seen in the Rose Quartz, the phenomenon is seen when the light source is behind the stone. The phenomenon is seen in crystals with milky and opaque appearance.
In short, di-asterism in gemstone is caused by a transmitted light, while epi-asterism occurs due star formation from a reflected light beam. Diffused light sources don’t form star shapes in the gemstones.
Asterism in Sapphires
The first instance of gemstone asterism was discovered by the Greeks. The cuts necessarily enhance the phenomenon. For example, star sapphires cut in cabochon display six-rayed star pattern. Other stones that exhibit the property are rubies, topaz, chrysoberyl and moonstone. The presence of titanium oxide particles, called rutile, gives the gemstones their trademark asterism.
Gemstones with asterism have always been linked to folklore and superstition. They were believed to ward off evils in the dark and keep the faith strong. Star Spinel, garnet, Diopside and sapphires were specifically cut to enhance the healing properties of the protective gemstones.
Asterism in Quartz
Pink and red quartz exhibit asterism that occurs due to transmitted light. Citrines also reveal asterism when heated to a certain temperature. The heating dissolves the finely lined rutile particles an align them to form stars. Round cut quartz are known to showcase exciting play of star formation, with interesting planes.
Asterism in Synthetic gemstones
Synthetic corundum is aluminium oxide which is largely produced to replace diamonds in cutting and abrasive tools. The asterism is seen in corundum gemstones.
Commercial influence on Star gemstones
Asterism is a key commercial feature that makes the gemstone look rarer in the market. Star beryl and star spinel gemstones are anyways rare, which are complemented with immaculate cuts and shapes. Star gemstones too have to meet the 3 C’s parameter to qualify as precious item: Colour, clarity and Carat.