The term alexandrite is applicable to the dark grass green, emerald green and bluish green varieties of chrysoberyl where the colours are attributed to the presence of chromic oxide in the composition. Chromic oxide in fact occurs as a trace element in the composition in negligible quantities and it is the presence of this element that helps in differentiating alexandrite from chrysoberyl. This means that for a chrysoberyl to be classified as an alexandrite, it should invariably have chromic oxide, the presence of which could be easily detected with the help of a spectroscope. Further, a simple visual observation to distinguish between alexandrite and chrysoberyl is to note its apparent phenomenon of showing a colour difference when observed under artificial light. In alexandrites the colour changes from green or bluish green in daylight to varying shades of raspberry red under artificial incandescent light. The colour under artificial light is usually inclined to violet. In some the overall effect could be very conspicuous while in others it could be less and in still others it could only be faintly apparent. However, it should be borne in mind that an apparent colour change could also be the effect of almost invisible brownish stains in the stones caused by iron. The only positive method by which to determine the difference is with the help of the spectroscope. This instrument will reveal the presence of chromic oxide if present by displaying a strong absorption doublet in the deep red end at 6803 Ao and weaker lines at 6650 Ao, 6550 Ao and 6450 Ao (Anderson, 1971). In certain instances reddish tints are quite visibly observed in normal daylight conditions as well. In such cases the colour change effect will quite naturally be enhanced under artificial light. By virtue of this colour change alone, these cannot, and should not, be identified as alexandrite unless the presence of chromic oxide is revealed on examination.
By way of a comparison a reference is made to alexandrite from Tanzania “which displays a blue green in white fluorescent light, rich green in daylight and a reddish violet with bluish overtones under incandescent light” (Dunn 1976).
Generally the average gem dealer is in the habit of identifying alexandrite only by the nature of the colour change. This would be extremely misleading as already mentioned since the presence of iron stains could produce a similar result. On the other hand it is well to remember that even if the stone does contain chromic oxide to fulfil the scientific classification requirement the expected colour change under artificial light could be extremely faint or hardly visible. The market potential of such material would be much less. The colours and colour change intensities could be of different degrees in different stones. This apparently is directly related to the amount and ratio of chromic oxide in relation to the other elements that are present in the composition. The colour-quality of alexandrite is judged by the nature and distribution of the original colour and the intensity of the colour change in artificial light. Accordingly the best quality alexandrite should be perfectly green or bluish green in daylight, changing to a raspberry red inclined to violet under artificial incandescent light.
Alexandrite is essentially a facettable gemstone but certain stones from Sri Lanka have been found to contain needle like inclusions which cause chatoyancy. To the observer these are visibly displayed in the form of ‘silk’. Chatoyancy in alexandrite is a rare phenomenon. Nevertheless such material of high quality is found among the gem gravels of Sri Lanka.
Generally large clear and flawless alexandrites are hard to come by. Good stones of appeal are extremely rare. Sri Lanka is singularly famous for having bigger sizes of quality grades. The Gem and Jewellery Authority of Sri Lanka has in its collection a facetted alexandrite of 30.77 carats. Another stone of very fine quality weighing 141.92 carats has been recorded from Sri Lanka. This has been seen and certified by the State Gem Corporation (now National Gem and Jewellery Authority) and it is presently in a private collection. This particular stone is a rare combination of size and quality. Information received states that this stone was processed off a rough that weighed 320 carats and was discovered from the gem gravels of the Ratnapura district.
An alexandrite of superlative quality is a gemstone of very rare occurrence and the countries known to produce such material are very limited. Apart from Sri Lanka alexandrites are also found in Russia, Myanmar (Burma) and Brazil. The Sri Lankan material occurs in colour varieties of green, greenish-blue and bluish-green. Generally it could be surmised that the green materials are mostly localized within the Rakwana, Bulutota, Deniyaya and Morawaka regions while others, the greenish blue and bluish green varieties, are more confined to the south and south west regions and the Matugama, Pelawatte, Bulathsinhala and Agalawatte regions.
It might perhaps interest the reader to note that alexandrite was discovered in the USSR in the year 1830 at a place in the Urals and this gemstone was named after Czar Alexander II of Russia. Strangely enough the colours red and green happened to be the Russian National Military colours, (Anderson 1976).