Julia Griffith FGA DGA EG, gemmology and diamond tutor at Gem-A describes the phenomenal appearances of Alexandrite, the June birthstone - 'Emerald by day, ruby by night'.
This extraordinary gemstone appears green or red dependent on the light that it is observed under – a colour change effect also known as the ‘alexandrite effect’. The rarity of this material and its chameleon-like qualities makes alexandrite one of the world’s most desirable gemstones.
In 1834, alexandrite was first discovered alongside beryls within the prestigious emerald mines in the Russian Ural Mountains. It is part of the chrysoberyl family, which is separate to the beryls, being a beryllium aluminium oxide as opposed to a silicate. Chrysoberyls have good durability and a hardness of 8.5 making them perfect for use in jewellery… that is, if you can find one.
Alexandrite is so rare that it has never truly been the main aim of commercial mining but instead a ‘by product’ as the likelihood of uncovering one is very slim. In Russia, just one crystal of alexandrite was found for every hundred-or-so emeralds.
Read more: Emeralds for Those Born in May
In the majority of alexandrites the clarity is poor – they are commonly riddled with fractures and appear translucent to opaque. Such specimens have little value and are often unusable as gems.
Rough specimens over 5 ct in weight are also a scarcity and most alexandrites found on the market are under 1 ct. Prices for fine specimens above 1 ct will rival or surpass fine quality rubies, emeralds and diamonds making them the one of the most expensive and rare gemstones in the world.
The most important factor for alexandrite is the quality of its colour change. The most prized colour change is a strong raspberry red in incandescent light and a bright green in daylight - however such a perfect specimen is not known to exist.
The colours seen in alexandrite are caused by chromium – the same colouring element which causes the red of ruby and the green of emerald. The amount of colour change seen is often given as a percentage - with a 100% colour change from one hue to the other being the most valuable.
The hues seen can also vary - if they stray too far from the expected colours of alexandrite or if the amount of colour change seen is minor, the value will be significantly affected and it is debatable to whether it can be classified as an alexandrite at all.
The colour change effect is due to alexandrite transmitting green and red light equally. Incandescent and daylight light sources are richer in different wavelengths (red or blue/green respectively) and this has a direct effect on what colour the gemstone appears to the human eye.
Alexandrites are also noted for their strong pleochroism. This is an independent optical effect from colour change, in which the gem will appear different colours from different directions. In alexandrite the pleochroic colours are green, orange and purple-red.
The finest quality alexandrites are said to be from the original deposits in Russia, which were mined out in the nineteenth century. ‘Alexandrite’ was named for the Russian Tsar, Alexander II. Legend states that this rare and beautiful stone was found on the day the heir became of age on his sixteenth birthday.
The vibrant red and green colours observed also mirror the colours of the national military of Imperial Russia and alexandrite became the official stone of the Tsardom of Russia.
Read more: Aquamarine for Those Born in March
According to Russian legend, wearers of alexandrite reap many benefits including good luck, fortune and love. A popular belief is that alexandrite helps the wearer strive to excellence bringing concentration, discipline and self-control.
Nowadays alexandrites are mined from Brazil, Myanmar, Tanzania, India and Madagascar. Sri Lanka has also produced some fine specimens - the world’s largest faceted alexandrite hailed from this locality and is a whopping 65.7 ct! A further rarity is chatoyant or ‘cats-eye’ alexandrites, which are cut in cabochon to reveal this optical effect.
Alexandrite has such extreme rarity that those seen on the market might not be quite what they seem. Alexandrite has been successfully synthesised in laboratories since the 1960’s and these synthetics have the same chemical, physical and optical properties of natural alexandrite and show a strong colour change - but lack rarity.
The most common simulant is synthetic colour change sapphire, which shows a greyish blue to pink colour change. At just a dollar or two per carat - it is extremely common on the market. This material has been made since 1909 so is often found in antique pieces of jewellery.
A good colour change, good quality, transparent natural alexandrite could easily cost the consumer a five figure sum per carat and beyond in regards to price. A fine price to pay for such a spectacular and exceptional gem. ■
This article was written by Gem-A for the May/June 2017 issue of The Jeweller - The magazine of the National Association of Jewellers
Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.
If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.
Cover image red square Moscow, Russia.