A star ruby or sapphire is a beautiful thing, but a natural, untreated one is a very rare thing. In this blog I will take a brief look at what these stones are, how they are formed and what to look out for.
Asterism is the optical effect that we are seeing in star rubies and sapphires. It is caused by the reflection of light which appears as a bright star on the surface of the gemstone. The light is reflecting from two or more sets of parallel fibrous or channel inclusions which are orientated within the stone. Chatoyancy, or, the 'cat's eye' effect, is the same thing happening with only one band of light visible which resembles the eye of a cat. Generally the inclusions are needles or fibres of other minerals such as rutile, or they may be tube-like cavities. The stone must be cut in cabochon in a particular orientation for the star to appear.
Corundum is the mineral family to which ruby and sapphire belong. When asterism occurs in corundum there will usually be three sets of parallel inclusions intersecting each other at 120° so that, when the stone is cut in cabochon with its base parallel to the plane of all three sets of inclusions, a six-rayed star will be seen. The best way to view the star is under a single bright light source such as a pen light, or the sun.
Synthetic star corundum
Just as synthetic rubies and sapphires are commonplace, so are star rubies and sapphires. Corundum was first synthesised back in 1837 by the French chemist Marc Gaudin, but it was Auguste Verneuil who perfected the flame fusion process, the first commercially successful method of producing synthetic rubies and, later, synthetic sapphires. The science behind all this is the topic for my blog, Synthetic Corundum, but by adding approximately 0.1 – 0.3% of titanium oxide to the starting material used to produce these stones, asterism will occur.
Differentiating synthetic star corundum from the real thing can be difficult for the untrained eye, but there are a few clues to look out for. Synthetic stones will often look rather too perfect – almost too good to be true. It is very rare to find a natural stone without some form of imperfection, whether that be in the form of additional inclusions, chunks missing from the stone, or the rays of the star being of uneven lengths or not quite straight. Sometimes the star will not even sit directly in the centre of the cabochon. Natural star stones also usually have coarser rutile needle inclusions. In a natural stone the star will appear to sit below the surface of the stone rather than directly on top when viewed under magnification. The cutter will also often cut the base of a natural stone in a domed shape in order to increase the weight of the stone, whereas a synthetic will usually have a perfect, flat base.
If you have any star rubies or sapphires that you would like to sell, please contact Simon Rufus, gemmologist at Grand Auctions, Folkestone, Kent for further details.
- Posted by Simon Rufus