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Asterism

An optical phenomenon displayed mainly in certain translucent to opaque Sapphires and Rubies, whereby a four or six ray star seems to appear beneath the surface of the gem, which will normally float across the surface of the gem as the light source moves.


To observe the effect it is best to view a gem under a single light source and slowly rock the gem. The star is caused by “tube like” fibrous inclusions, which are all arranged parallel to one another. Prior to shining a light source on the gem, it looks quite normal; simply a regular coloured opaque gem. But as soon as the light is applied, it reflects off the tips of the inclusions and the star is revealed.

To maximise the star, the lapidarist will cabochon cut the gem.  Prior to making their first cut, they will study the gem to predict where the asterism will take place, trying to ensure that it appears as close to the top of the dome as possible. Possibly the most famous of all Star Sapphires is the Star of India. Weighing a huge 563 carats, it is a stunning gemstone and on a recent visit to the Natural History Museum in New York, I was amazed at how long I had to queue just to walk past this star attraction. The gem was donated to the museum in 1900 by J P Morgan. In 1964 the gem was stolen from the museum by an infamous high profiled burglar known as “Murph the Surf” (his real name was Jack Murphy). Luckily for the museum, highly embarrassed by their breach of security, the $400k gem was later recovered in Miami. 

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A Sri Lankan Star Sapphire showing six rays.

A pair of Burmese Star Rubies.