Have you ever been mesmerised by the play of light within a gemstone? Or transfixed by an optical effect that makes you constantly move the jewellery, the effect seemingly floating across the surface of the gemstone? How did nature produce such as marvel? Then you need no introduction to star gemstones, which display what is officially known as Asterism. (Think of the typographical symbol, the asterisk * taken from the Ancient Greek ‘asteriskos’ meaning little star. Not to be confused with Asterism in astronomy – meaning “a prominent pattern or group of stars that is smaller than a constellation”.)
To get the science bit out of the way – first you have to understand chatoyancy, which is a specific type of optical phenomena seen in certain gemstones including Sapphire, Ruby, Chrysoberyl and Diopside. It is caused by the play of light that reflects off parallel sets of fibrous inclusions within the stone – which are arranged neatly in perpendicular harmony, at an atomic level. To explain this more visually, imagine a cotton reel of shiny yarn and the band of light that reflects from the curved surface.
To view this effect at its best, the stone must be cut en cabochon – a smooth domed shape that optimises the curved surface of the gemstone and allows for the resulting play of light. A single band of light is known as a cat’s eye (Chatoyancy comes from the French ‘chat oeil’ or cat's eye). In asterism you have several bands of light crossing each other, usually two or three, to form either a four pointed or six pointed star. It is possible (but very rare) to see 12 pointed stars.
For optimum effect, it is best to view the stone under sunlight or a single source of light. Multiple light sources can render the effect fuzzy or unfocused. In fact, under fluorescent lights or cloudy conditions the effect might be hard to spot altogether.
The most famous Star Ruby is the Rosser Reeves Ruby, weighing in at an impressive 138.7 carats. Currently housed in the Smithsonian Institution in Washington, it was recut from its original 140 carats in order to centre the star effect.
Larger still and staggeringly, nearly the size of a golf ball, is the 'Star of India' (above). The world’s largest gem quality blue-grey Star Sapphire, weighing a staggering 563 carats. If you are ever in New York, pop into the American Museum of Natural History and have a look.
Shop our Star Sapphire and Star Ruby collections.
MORE GEMSTONE PHENOMENA
Fluorescence and Phosphorescence