Alexandrite - the final June birthstone

Alexandrite - the final June birthstone

Alexandrite has often been referred to as "emerald by day, ruby by night," thanks to its unique colour shifts between shades of green and red. The closer the colours are to pure green and red, and the greater the contrast between the two colours, the higher the value of the stone. The transition between the two colours is measured as a percentage and ranges from 5% to 100%. Alexandrite is an extremely rare and expensive species of gem-grade chrysoberyl, with some exceptional varieties achieving over $75K per carat but most fine examples range from $2.5-$15K per carat for smaller stones. The less valuable examples tend to display blue-green and purplish or brownish red colouration. After colour, clarity is the next factor in determining the value of an alexandrite. As with most gemstones, the most commonly found alexandrite cannot be faceted and is mostly cabbed (cut into cabochons). Fine examples of more than one carat are extremely rare, accounting for the exponential hike in prices past this point. At 8.5 on the (1-10) Moh's scale of hardness, alexandrite sits slightly above emerald at 7.5-8, not far behind and corundum (ruby and sapphire) at 9, with diamond at 10. Alexandrite was first unearthed in the Ural Mountains of the Russian Empire in 1830 by the Finnish mineralogist Nils Gustaf Nordenskiöld, who initially misidentified the stone as emerald. It derived its name in honour of Czar Alexander II, courtesy of Count Lev Alekseevich Perovskii.

The royal connection has added to the mystique and desirability of the stone throughout the last two hundred years. In the late nineteenth and early twentieth century the legendary French jewellery house Fabergé were commissioned by the Czars and the Russian nobility to create some amazing pieces containing very fine alexandrite and this helped to fire the public imagination. However it wasn't until the 1950s that it began to be recognised as a June birthstone, alongside moonstone and pearl. Alexandrite is so rare because in order for it to be created chromium (Cr) must combine with beryllium (Be), one of the scarcest elements on the planet, deep within the earth's crust. To make matters worse, Be and Cr are only found together in a very select few locations in the world. Consequently most of the original alexandrite mines were depleted by the end of the nineteenth century. Despite some new discoveries in the past 30 years or so, alexandrite is still one of the rarest precious gems on the market. As a chrysoberyl, very rare (and expensive) alexandrite can exhibit a chatoyancy, or "cat's eye" effect when cabbed (a unique optical effect the stone produces). The type of light surrounding an alexandrite can accentuate its colour change so gemmologists assess the stone's colour according its colour in natural daylight and in a darkened environment, so as to provide a suitable benchmark comparison between the two extremes. However, differing types of light can result in other colours being displayed so the change in colour is not a binary one.

(A beautiful Fabergé alexandrite and diamond ring) 

Alexandrite also displays subtly different colours according to the provenance (or geographical origin) of the stone. Brazilian examples are usually paler in colour but some recent specimens have been discovered with much more vivid saturation. The difference in the colour of the Brazilian stones has been put down to the presence of the element gallium (Ga) and the absence of aluminium (Al) in their chemical composition. Zimbabwean alexandrite display a vivid verdant green in sunlight and deep 'ruby' red in low light but each stone tends to be very small in size and nearly all are under one carat. Due to the high value of alexandrite, significant volumes of synthetic substitutes have been lab 'grown' over the past 50 years or so. These substitutes have the same chemical and physical characteristics as their natural counterparts so it is true to say they are real alexandrite but they are not natural alexandrite. The close similarity does mean that synthetic alexandrite are amongst the most valuable of all artificially created gemstones. A trained gemmologist can however discern a synthetic stone from a natural one, with the aid of laboratory equipment, by way of the analysis of the inclusions. Some entirely fake alexandrite can be found, largely in the form of synthetic corundum that exhibits a colour change similar to that of chrysoberyl species and these can be produced very cheaply so it pays to look for stones that have been certified by a recognised institution.

(Some very rare cat's eye alexandrite)

 It is also important to note that while alexandrite is a member of the chrysoberyl family, not all colour-changing chrysoberyls are actually true alexandrite. As is so often the case with coloured gemstones, the dividing line between one type and another can often be slightly blurred and open to interpretation (in a similar way to pink sapphire and ruby for example). Natural alexandrite are not usually treated or oiled in any way. Although some of the original mines in the Urals have re-opened, production is very low and most new finds have come from Brazil, Madagascar, Burma, Sri Lanka and Zimbabwe. The Urals remain the source of the finest quality alexandrite ever discovered. The Smithsonian contains the largest known faceted example, at 65.7 carats, which is a green-red colour change stone from Sri Lanka but the largest known Russian alexandrite is 30 carats and the overwhelming majority are under a carat. The British Museum houses a 43 carat example and stones of over 50 carats are thought to reside in several private collections. Due to its hardness, alexandrite is ideal for jewellery but care must be taken when faceting to avoid cleaving the stone and it can crack or chip under some circumstances, including conditions of extreme heat. Although it is possible to clean alexandrite mechanically, it is advisable not to do so unless you are an expert. It is far safer to use warm, soapy water and a clean, soft cloth to ensure you do not have any permanent effect on the surface of the stone.

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