Alexandrite is a unique coloured gemstone that displays an extremely rare colour transformation from green in daylight or fluorescent light to brownish- or purplish-red in the candle flame or incandescent light. Being regarded as “emerald by day, ruby by night”, alexandrite is one of the most phenomenal gemstones available today.
Alexandrite History and Origin
Alexandrite is an extremely rare variety of chrysoberyl. Despite its name chrysoberyl, this gem does not belong to the beryl mineral group, but rather, it is classified as an independent mineral group.
The history of alexandrite dates back to the times of Imperial Russia. Being discovered by a Finnish mineralogist and traveller Nils Gustaf Nordenskiöld in 1834, alexandrite was initially thought to be an emerald as it was found in emerald mines located in the Ural region, Russia.
Receiving the mineral sample for examination from Lev Perovskii, Nordenskiöld was confused with the high hardness of the gem, 8.5 on the Mohs scale, while emerald is 7.5 – 8. The same evening, while looking at the gemstone under candlelight, he was surprised to discover that the stone had changed its colour from green to purplish-red. Later, the scientist confirmed the discovery of a new variety of chrysoberyl and suggested to name it “diaphanite” (from the Greek “di” two and “aphanes” meaning appear or show).
However, Perovskii had his plans. He used the gemstone to integrate himself with the Imperial family by presenting it to the future Tsar and naming the gemstone alexandrite in honour of Alexander II of Russia. Inevitably, alexandrite was declared the official gemstone of Imperial Russia’s Tsardom.
Although the Ural region in Russia was thought to be the only source for large alexandrites for quite a long time, large specimens were discovered in Minas Gerais and Brazil. Other sources for alexandrite include Madagascar, India, Tanzania, Sri Lanka and Myanmar (Burma).
The colour changing phenomenon seen in alexandrites is referred to as the “alexandrite effect”. The colour change is the result of the complex way the mineral absorbs light and it can be observed under certain lighting conditions, such as daylight or fluorescent light and candlelight or incandescent light.
Alexandrite is a strongly pleochroic gem, which means it can display different colours when viewed from different angles. Depending on the direction the stone is viewed from, it can show green, purple-red, orange and yellow colours. However, the colour changing properties of alexandrites are completely independent of the pleochroism.
Commonly alexandrite exhibits emerald green colour in daylight and purplish-red under incandescent lighting. However, it can also occur with yellowish or pink colours. Some rare specimens can exhibit chatoyancy effect when cut into cabochon.
The green colour in alexandrite is the result of chromium impurities. Chromium is the same colouring element that causes green in emeralds.
Alexandrites from Sri Lanka are known to exhibit a khaki to brown colour range, while the ones from Zimbabwe usually display little colour change and typically come in darker colours with tints of purple. Tanzanian specimens tend to occur in lighter tones. Brazilian alexandrite is known for its blue to purplish colour change. The most valuable alexandrites are those with pure hues and a strong colour changing effect.
Alexandrite is a clean gemstone, meaning it contains few inclusions. In its rough form, it can range from transparent to opaque. Once cut and polished alexandrite displays vitreous lustre.
There is a dramatic rise in price for clean alexandrites with good colour change and strong colours, especially for stones over 1 carat in weight. Fine quality alexandrite is considered to be more valuable than blue sapphire, emerald and ruby.
The stones that exhibit chatoyancy or cat’s-eye effect are even more valuable.
Alexandrite Cut and Carat Weight
Alexandrite is rarely found in large sizes, any stones weighting more than three carats are considered to be extremely rare and valuable as a result.
Alexandrites are most commonly fashioned into traditional shapes such as round, oval, cushion, pear and marquise cuts. In case the rough material possesses chatoyancy properties, it is cut into cabochon.
The pleochroism effect in alexandrites makes it a challenge for cutters as they need to orient the gem to show the strongest colour change through the crown. It is crucial to position the rough so the fashioned stone shows both green and purplish-red colours face-up.
|Chemical Name||Beryllium aluminium oxide|
|Colours||Green, yellow, brown, multicolour|
|Hardness||8.5 on the Mohs scale|
|Refractive Index||1.74 - 1.76|
|Specific Gravity||3.70 - 3.78|
|Transparency||Transparent to opaque|
|Birefringence||0.007 - 0.011|