|Alexandrite An Emerald by Day, An Amethyst by Night
. The year was 1830, when
Ekaterinburg (named for the Russian Empress Catherine II) was a bustling
city on the route from Russia to Siberia. The date was April 23, the
date of the young Czarevitch Alexander Nicolajevitch's birthday, when
as legend goes, a new gem was discovered and created a sensation.
A variety of chrysoberyl was found that was bright green in daylight
and rich red when seen under artificial light at night. Now red and
green were also the colours of the Russian Imperial flag and thus
the gem was named Alexandrite by mineralogist Nordenskjold in honour
of the future ruler of the Russian Empire who would go on to rule
as Czar Alexander II from 1855-1881.
High quality, large alexandrite specimens are rare and very expensive and are mostly seen at auctions and estate jewellery sales. What makes this gem so valuable is its three colour factors: daylight colour, incandescent light colour, and the degree of change between two. Top quality alexandrite is ;reen to bluish green in daylight md red to purplish red in ncandescent light. These dual :olours led pioneering gemologist fax Bauer to famously describe the Jralian alexandrites as "an emerald by day, and an amethyst by night." On the surface, a colour :hange gem does not seem all that great, since there are colour change sapphires also and other gem species jihat boast a colour change variety. However, when you dig deeper you ivill find that most of these gems that exhibit a colour change capability psually change only to the next shade on the colour wheel i.e. the colour that comes next to it, so blue nay become purple and red may turn to violet but alexandrite doesn't display this subtlety and just jumps from red to green across a wide variety of hues in between. Not only that, the degree of change is almost absolute in the best specimens with no bleeding in between of the colour before.
How does alexandrite manage this alchemy? Are there two different colouring agents within the stone that prevail over the other under different conditions? No, the red and green colours in the stone both owe their bright hues to the presence of a single trace element chromium. What actually happens is that the chromium present in alexandrite is of a variety that it responds to different wavelengths of light and changes colour accordingly. Fluorescent light and daylight is rich in blue wavelengths while incandescent light has more red wavelengths so that explains the different colours in different lighting conditions. The finest daylight colours of alexandrite are rich bluish green, the green colour predominating with only a hint of the bluish tint. The best nighttime colours of alexandrite are pinkish-purplish red to purplish red, however the trace of pink is less desirable. Stones from Tanzanian sources exhibit a purer green in their daytime colours while under incandescent light they tend to be more purplish than red. Tanzanian alexandrite can sometimes exhibit both the day and night colours simultaneously when viewed under a light bulb if there is diffused daylight present and although technically, this is a fault in the stone, it actually adds to its attraction and appeal.
Having discussed the colours of alexandrite at length, it is necessary to point out that like in most rare and expensive stones, these are the ideal colours and are thus more difficult to find. Except for the best pieces, most natural alexandrites actually are not pretty to look at Grey is predominant and brown is often found as a saturation modifier or mask. Brazilian and Madagascan stones tend to be greyish while Tanzanian specimens are more on the brown side. In fact, stones of more than five carats in size that do not show a mask are very rare and thus all the more valueable.
Another problem is that colour saturation is not very deep in most stones and the vast majority of alexandrite, regardless of its source, will occur in paler tones than are considered ideal and will also display a very weak colour change. Those that do show a strong degree of colour change will tend to have poor crystal that will give them an overall murky appearance. The best tones for alexandrite are said to fall between 70 to 75%. Eye clean stones with tonal values close to 70% are more transparent, have better crystal and exhibit colour changes that are breathtaking. That is one of the telling points: the more transparent the stone, the more visible the colour shift is bound to be.
The darker toned stones from Brazil rarely show good crystal while the Tanzanian gems have more transparency and exhibit better crystal than the Brazilian specimens, The funny thing is that although the finest Tanzanian alexandrite can rival the best found anywhere, they are still valued less highly than the Brazilian alexandrite. In the pecking order of value though, it is the Russian stones that rule the roosi and collectors are always willing to pay a premium for alexandrite that traces its origin to Russia. Generally, alexandrite is found in smallish crystal sizes and are rarely free from inclusions. Negative crystals and parallel rutile silk are common inclusions. As clean alexandrites cross the 3-carat mark, the prices begin to go through the roof. The largest alexandrite was found in Sri Lanka, 1,876 carats in the rough, while the largest cut stone is of 66 carats and is on display at the Smithsonian Museum in Washington D.C. In fact, although it ranks with ruby and padparadscha as among the world's most desirable gems, this is generally a collectors' stone as there are too few fine quality natural specimens to be found and most of the alexandrite in the market today is synthetically created.
Although natural alexandrite is not usually subjected to any colour or clarity enhancement techniques, synthetic versions of the gem do exist and are often difficult to tell from the natural stone. Alexandrite was first synthesised in 1973 and Chatham created alexandrite was synthesised in 1975. Often it is also imitated by synthetic corundum (mostly synthetic colour change sapphire coloured by vanadium) and synthetic spinel. The synthetic versions change from grey/blue violet in daylight to a reddish violet in incandescent lighting. Synthetics also tend to have flux inclusions, triangular metallic platelets, curved striae, or gas bubbles, depending on the technique used to create them. Synthetic alexandrite also tends to have a slightly lower refractive index, and exhibits stronger fluorescence than the natural gems.
Alexandrite is the rarest variety of chrysoberyl (BeAl O ), which occurs in granitic pegmatites and mica schists. Small scale replacement of alumina by chromium oxide results in the formation of alexandrite and is also responsible for the stone's dramatic colour change. Two very rare varieties of alexandrite are the cat's eye alexandrite and the star alexandrite. Alexandrite has a high degree of hardness, 8.5 on the Moh's Scale. It can thus weather daily wear but it should be protected from sharp blows and scratches, as well as from harsh chemicals and excessive heat. Therefore, it makes sense to be careful about getting alexandrite remounted as heat from the jeweller's torch may have an adverse effect on the colour change. It can be cleaned in ultrasonic cleaners but not for more than 3-5 minutes. Alexandrite has a specific gravity of 3.74 and a refractive index of 1.746-1.755. Its crystal system is orthorhombic.
Since the cost of rough alexandrite is very high, the polisher will try to save as much weight as he can and thus the final shape of the faceted stone will really depend on its form in the rough. Therefore alexandrite is usually cut in the cushion shape or ovals and emerald and heart shapes are also quite common thought the weight wasting round cut is not normally to be encountered.
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