Alexandrite

Revision as of 15:07, 7 August 2013

Alexandrite

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. It is type II gemstone, and can be expected to show some inclusions to the unaided eye that don't detract much from its beauty, although many cut stones are 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. Many English Victorian designs contain alexandrites, generally smaller stones. Alexandrites 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. It is difficult to find matching alexandrites for earrings, three stone rings and necklaces that share the same shape, color, size and color change.

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.

Sources

Current sources of Alexandrite include Russia, Brazil, Sri Lanka, Myanmar, Tanzania, Zimbabwe, Madagascar and India

Russia

Original source of the Alexandrite. Gemstones are very high quality with distinct color changes and saturated colors. These mines produced many of the larger Alexandrites on the market weighing 5 carats or more. Mines have since dried out but still continue producing alexandrite in low volume.

Brazil

Also produced large specimens of 5 carats and up. Hematita is still one of the most important deposits of alexandrite, although production has decreased in resent years. Its stones show a nice color change from bluish green to raspberry red. Occasionally stones with chatoyancy are discovered in Brazil, mainly around the Americana and Santana valleys, and are cut as cabochon cat's eyes.

Sri Lanka

Sri Lanka is known to produce the cleanest alexandrites, often clear of any visible inclusions but generally lacking in their hues. They are often larger then Russian gems but their colors are less desirable, appearing yellowish green in fluorescent light and a brownish red in incandescent light.

East Africa

Alexandrites have been mined in the Tunduru area of southern Tanzania since the mid-1990's, they have produced some fine specimens over the years. Other sources are Madagascar and Zimbabwe.

Burma / Myanmar

India

Found mostly in the Andhra Pradesh area.

Buyers Guide

Distinct color change is the important factor when purchasing an alexandrite. Top quality alexandrites show a strong color change from purplish red in incandescent light to bluish green in daylight, without any trace of brown or gray and have moderately strong to strong color saturation. Such stones command heavy prices so any inexpensive gems with these colors is likely a synthetic or simulant. For a significant purchase, a certification of gem identity and natural vs. synthetic is recommended.

Certification

GIA offers identification reports for loose or mounted Alexandrite but no grading as is their policy with all other gemstones. Their cert also states whether the alexandrite is natural or synthetic, and lists any detectable treatments as well as providing the weight, measurements, shape, cutting style and color of the stone.

Alternatives

Alternatives to the alexandrite are color change garnet and color change sapphires. Look alike imitations are synthetic color change sapphire and synthetic color change spinel.

Birthstones & Anniversaries

Alexandrite is a June birthstone, along with the moonstone and cultured pearl. It is also the 55th wedding anniversary gemstone.