Wednesday, January 19, 2011

Asteriated Sapphires (Star rubies and star sapphires)

Among the corundum family are also asteriated rubies and sapphires as mentioned earlier. Asterism is a star like reflection effect caused by certain minerals within the host corundum. These are microscopic acicular mineral inclusions of special orientation. Very often these mineral inclusions are rutile (TiO2) in the form of crystallized needles. Asterism in corundum is more common and more effective than in most other minerals that do display this phenomenon. The inclusions are oriented within the stone in a particular relation to the vertical axis of the corundum crystal (c-axis). Such stones, when cut 'en cabochon', display a special reflection effect in the form of a six, or in rare instances a twelve rayed star on the cabochon surface. An interesting and common observation is, that in the case of a twelve rayed star, the 'silk' that produces it is bronze coloured causing a 'schiller' effect at the base of the stone. The asterism or the star effect is best highlighted when observed under an overhead light source. In this connection it is interesting to note the reference made by Paul Desautels of the Smithsonian Institute (1965) to a four starred sapphire in the Institute's collection. This gem is said to be from Sri Lanka.

Theoretically, asterism could occur in any colour variety of corundum but this phenomenon could be most highlighted and made conspicuous only in the colour varieties which could in effect create a colour contrast to the silvery reflection effect of the star. The ideal stone should also be semi-transparent, translucent or opaque. The star effect is most attractive in rubies and sapphires of darker hues. Asterism has also been seen in yellow and brownish yellow sapphires but the colour contrast is so little that the star effect is not prominent. In the ideal stones the rays should be properly centered showing uniformity of the angular intersections and the rays should be perfectly straight, each ray extending more than half way down towards the girdle and the individual rays narrowing to an end. The rays should be sharp and well defined, rather than being broad, and should merge imperceptibly into the stone. The rays should be perfectly white or silvery without being affected by the colour of the stone. The stones are best if these do not display colour banding or zoning when viewed from above. In some, immobile 'silk' (geuda) is seen as whitish patches scattered within the stone. These spoil the appearance of the stone. Accordingly a top quality stone will carry a sharply defined ray against a pleasing colour, and such stones are most favoured. The combination of such features are considered rare. Nevertheless, Sri Lanka is singularly reputed for having asteriated corundum which combine these features.

Although certain rough material from certain localities in Sri Lanka show clear indications of asterism, these do not produce this desired result when polished for no explicable reason. This is perhaps a locality trait. Instead this material may sometimes display 'spot' asterism, which is comparatively not of much value in trading circles. Quite many rough stone dealers have experienced this and if the rough stones are known to be from any such known areas the dealers exercise tremendous caution.

It is said of asteriated corundum that, a combination of a perfect body colour, the ideal degree of transparency and a perfectly sharp and centered star is a rare occurrence in nature. A star sapphire of blue colour and of exquisite beauty from Sri Lanka weighing 392.92 carats is presently in the collection of the National Gem and Jewellery Authority.

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