Alexandrite
Alexandrite
Contents |
Phenomenon
Alexandrites are renown for their color change properties ('alexandrite effect'), they have strong pleochroism and occasionally display chatoyancy (cat's eye effect).
Color Change
Only chrysoberyl that contain distinctly change colors can officially be called an alexandrite in the jewelry industry. When viewed in in natural daylight or fluorescent light these gemstones appear green. When viewed in incandescent light or a candle flame they appear red. So striking is the transformation by the Alexandrite above other color changing gemstones, that this phenomenon is referred to as the 'alexandrite effect' in the world of gemology. Alexandrites are pleochroic gemstones but their color change properties are totally independent of this, instead resulting from their unusual light-absorbing properties that cause a strong absorption of light in the blue and yellow portions of the color spectrum. Color change in Alexandrites are caused by minor traces of chromium ions that replace aluminum in the crystal structure. Occasionally vanadium also takes part in the process. Some alexandrite owner enjoy flicking the gem's colors back and forth by rotating it between fluorescent and incandescent light sources.
Pleochroism
Aside from its unique color changing abilities, Alexandrites are a strongly pleochroic gem, displaying different colors when viewed from different directions. Depending on the angle viewed, alexandrites can display various shades of orange, red, yellow and green. Its pleochroic properties are completely independent from the 'alexandrite effect'. Typically the three pleochroic colors seen in an Alexandrite are are orange, green and purple-red.
Cat's Eye
Extremely rare specimens of Alexandrite contain long, thin inclusions that are aligned parallel to each other, causing it to exhibit chatoyancy (cat's eye effect) when the stone is cut into a cabochon. Cat’s-eye alexandrite generally originates in Brazil and is more valuable then its standard form.
Details
Color
Shades and hues of the Alexandrite vary widely and are often influenced by their mining origin. Alexandrites most treasured colors are green by daylight and red by incandescent light, with pure hues that display a strong color change, as was prevalent in many of the gemstones originating from the initial Russian find. Brazilian alexandrite is known to be highly saturated and display a bluish green to purplish red color change. Alexandrite originating in Zimbabwe often have very little color change and are typically darker in color with tints of purple. Sir Lanka alexandrites have a muddy appearance and exhibit a khaki to brown color change. Specimens from Tanzanian tend to have lighter tones and possess decent color change. Other varieties of alexandrite can appear yellowish or pink in daylight and turn raspberry red by incandescent light. As a general rule; Alexandrites that are too light lack the intensity found in high quality gems. Stones that are too dark lack brightness and can appear almost black.
Sizes
Most alexandrite are under a carat, anything over 3 carats is considered rare. Originally all alexandrites over 5 carats were assumed to have originated in the Ural Mountains of Russia but larger sizes have since been found in Brazil. To date the largest cut alexandrite weighs 66 carats and is part of the gem collection at the Smithsonian Institution of Washington, D.C.
Cuts
Alexandrites are often cut in traditional shapes such as round, oval, pear, marquise and cushions, but they can also be found in fancy shapes such as trillions and heart cuts. Due to their rarity and value they are often faceted as mixed cuts that have have brilliant-cut crowns and step-cut pavilions in order to retain weight from the rough. Alexandrite’s pleochroic properties makes it a challenge for cutters who must orient the gem to display the strongest color change through the crown. Alexandrites are almost never cabed unless they contain chatoyant properties, in which case they are cut as cabochons to create a 'cat's eye'.
Clarity and Luster
Alexandrite rough ranges from transparent to opaque. Most cut stones are typically clean. Once cut and polished, alexandrites exhibit a vitreous luster.
Varieties
Varieties of alexandrite include 'cat's eye alexandrite', 'pink alexandrite' and 'yellow alexandrite'.
Treatments
Alexandrite gemstones are typically untreated, very rarely they may be dyed or oiled.
Stability
Alexandrite is a hard and durable gemstone that survives well in jewelry and is stable under normal wearing conditions. It has a hardness on 8.5 on the mohs scale, excellent toughness, remaining stable in conditions of heat and light and has no reactions resulting from common chemicals.
Care
Alexandrites are easy to care for and can easily be cleaned using warm water and soap. It is generally safe for them to undergo ultrasonic and steam cleaning unless they contain fractures, in which case it is not advisable. It is best to avoid cleaning an alexandrite with bleach or any other harsh chemicals. Since alexandrites are harder then most gemstones, they should be wrapped with a protective cloth when sharing a jewelry box with softer stones, to avoid scratching them.
Usage in Jewelry
Alexandrites are hard and durable making them ideal for any jewelry design such as earrings, rings, brooches, pendants, pins and pendants. They are often found as accent stones since they are small and expensive. Original alexandrites from Russia's Ural Mountains are typically center stones and are often found in estate jewelry.
Synthetics
Synthetic alexandrite has been created since the 1960's using both the flux and czochralski methods. It is quite expensive to manufacture synthetic alexandrite, so even lab-grown stones carry high price tags. Synthetic alexandrites are marketed using the name of the company who created it along with an indication that the stone is synthetic, although the actual term 'synthetic' is hardly used. Ex: Chatham Created Alexandrite, Chatham Laboratory Grown Alexandrite, Chatham Laboratory Created Alexandrite.
Simulants
Common simulants for Alexandrites are color change garnet, synthetic color change sapphire and synthetic color change spinel. Synthetic color change sapphire is often infused with either chromium or vanadium to display a colors similar to Alexandrites.
Alexandrite Identification
An Alexandrite can easily be separated from most stones through its color change properties. A simple flicker between fluorescent and incandescent light sources does the job. This leaves only color change garnets, synthetic color change sapphire, and synthetic color change spinel. After a stone has been identified as an Alexandrite it still has to be determined if it is a synthetic or natural.
History
Alexandrites were first discovered in April 1834 by Finnish mineralogist, Nils Gustaf Nordenskjold. First specimens of the stone were found in emerald mines located in Russia's Ural Mountains region, near the Tokovaya River. First thinking it was an emerald, Nordenskjold later identified the gem as a chromium bearing, color change variety of chrysoberyl. Count Lev Alekseevich Perovskii named the gemstone 'Alexandrite' in honor of the future Russian Tsar Alexander II. Some legends claim it was discovered on his 16th birthday. Since Alexandrites are colored both red and green, the main colors of old Imperial Russia, it became the official national gemstone.
Stones from the Ural Mountains were of very fine quality, displaying vivid hues and strong color changes. When the Russia mines dried up after a few decades of production, the alexandrite almost lost mention in the jewelry industry. Things changed dramatically in 1987, when alexandrites were uncovered in the Hematita region of Minas Gerais, Brazil, once again giving the gemstone a boost. Although they did not compete with the quality of the Russian material, Brazilian alexandrites did show a distinct color change along with good clarity and color. Later deposits were found in Sri Lanka, Myanmar, Tanzania, Zimbabwe, Madagascar and India. Some of the new stones were high quality but many had less defined color changes and muddier hues.