Gemstones with Asterisms and Chatoyancy

When some gemstones are cabochon cut, when the stone is domed and polished rather than facetted and often used in engagement rings; they can show Asterism or Chatoyancy. The name Chatoyancy coming from the French to mean a cat’s eye is when a parallel ray, that bares resemblance to the slit in a cat’s eye, is seen on the surface of the stone. When the stone shows four or more ray this is known as asterism or a star.

The way in which this cat’s eye or star is produced is down to light reflecting off of the internal features of the stone. This could be a type of inclusion which is fibrous or needle-like, these are often Rutile inclusions, or even cavities within the stone. What defines the Chatoyancy or asterism is the number of inclusions that the light has been reflected from. For example if the light is reflected by only one inclusion it will produce two parallel rays thus showing the Chatoyancy or cat’s eye. However when the light is reflecting from two or more inclusions that overlap an asterism or star is seen on the surface of the stone. If the star has four rays then the light has been reflected from two strands of inclusions, if a six rayed star appears on the stone then this will have been reflected from three strands and so on.

There are several types of gemstone that show a cat’s eye but the term is synonymous with the Chrysoberyl, the other types of stone which display a cat’s eye have the term “cat’s eye” added in front of their name such as cat’s eye apatite. Cat’s eye can be found amongst finer variants of gemstones such as tourmaline, emeralds, aquamarines and tanzanite’s, to name a few, but this occurrence is quite rare. They are much more commonly found in several types of quartz, most famously tiger’s eye.

Asterism can be found in corundum, all gem-quality corundum is either a sapphire or a ruby. The fibres of Rutile inclusions which cause the appearance of the star are often known as “silks”. Star sapphires and rubies are very collectable and rare, a good quality stone would have a star where the rays are sharp, unwavering and reach girdle to girdle. Due to the Rutile inclusions the stones can appear milky, so a star sapphire or ruby that is clear and has a good depth of colour is also an important factor when it comes to ascertaining the quality of the individual stones. Sapphires come is many colours with many variants of shade, this is also true of star sapphires, from blues to green, lilac to pink, yellow to orange and grey to black. When the colour is a definite shade of red this stone is then considered to be a ruby.