Alexandrites are also obtained from sources in Sri Lanka, but the hue of these stones compares less than favourably with that of the Uralian alexandrites.
They appear green in daylight and a brownish red in artificial light. The Tunduru area in southern Tanzania has also produced some outstanding specimens since the mids. Alexandrites are also found in India, Burma, Madagascar and Zimbabwe. Although this stone is still considered a rarity, specialised gemstone dealers do stock it, especially since improved trade relationships between Russia and the rest of the world have ensured a better supply of Russian alexandrites to the market.
Often exhibits parting along twinned crystals. Mineral Class Chrysoberyl. Newer Post Older Post Home. Chemical Formula. BeAl 2 O 4. Crystal System. Refractive Index. Alexandrite's characteristic green to red color change results from small scale replacement of aluminium by chromium oxide. Natural alexandrite is very rare. The finest alexandrite crystals ever found came from the Tokovaya river deposit, and the greatest alexandrite specimen ever found is housed in Moscow's Fersman Mineralogical Museum.
Alexandrite has a hardness of 8. Alexandrite has a refractive index of 1. The refractive index RI , measured using a refractometer, is an indication of the amount light rays are bent by a mineral. Birefringence is the difference between the minimum and maximum RI.
When birefringence is high, light rays reflect off different parts of the back of a stone causing an apparent doubling of the back facets when viewed through the front facet. Most gems have a crystalline structure.
Crystals have planes of symmetry and are divided into seven symmetry systems. The number of axes, their length, and their angle to each other determine the system to which a crystal belongs. A distinct color change is the primary qualification for a chrysoberyl to be considered alexandrite. Alexandrite is a trichroic gemstone which may absorb and reflect light differently in each of its three optical directions.
However, it is not the trichroism that is responsible for the remarkable change. In rubies the chromium absorption band is around nanometers and in emeralds, the band is around nm.
In alexandrite, where the band is at nm and right between ruby red and green emerald, the stone is balanced between them. When the light is balanced daylight , the stone will be green but when the light source is reddish incandescent , the stone appears red.
Daylight, is more equally balanced. Since our eyes are most sensitive to green light, the balance is tipped to the green side. The strength of the color change is related to the difference in the areas of transmission, relative to the absorption in the yellow.
The greater the difference, the stronger the color change. When evaluating alexandrite, pay the most attention to the color change: the more dramatic and complete the shift from red to green, without the bleeding through of brown from one color to the next, the more rare and valuable the stone.
For alexandrite, the quality of the color change is paramount. While the holy grail is a gem whose color changes like a traffic light from green to red, such a stone has yet to be found. In fine examples, the change is typically one from a slightly bluish green to a purplish red. The other important value factors are the attractiveness of the two colors - the more intense the better - the clarity, and the cutting quality.
Because of the rarity of this gemstone, large sizes command very high premiums. Offer Ends Nov Skip to main content. AJS Gems. Alexandrite Gemstone Information.
Click here to see all our Alexandrite for sale. There are many reasons, but mainly it comes down to value and choice Color Color Change Refractive Index 1. In the market, alexandrites are found in a variety of shapes and cutting styles. Ovals and cushions are the most common, but rounds and emerald cuts are seen as well, as are fancy shapes.
Alexandrite is generally not treated, although the occasional stone may be treated by oiling or dying. Like many other gemstones, alexandrite emerged millions of years ago in a metamorphic environment. But unlike many others, its formation required specific geological conditions. The chemical elements beryllium a major constituent in chrysoberyl and chromium the coloring agent in alexandrite have contrasting chemical characteristics and do not as a rule occur together, usually being found in contrasting rock types.
Alexandrite gemstones are not treated or enhanced. A synthetic form of Alexandrite exists, and it can be difficult to distinguish natural and synthetic Alexandrite from each other. Most synthetic Alexandrite is in fact synthetic Corundum or synthetic Spinel with added vanadium or chromium to induce the color-change habit. However, the Russian deposits have long since been exhausted, and Russian sources of Alexandrite are near impossible to obtain.
Small deposits were subsequently found in Sri Lanka Ceylon , and until the 's these were the only two sources of this rare gemstone. Since then, new deposits have been discovered in Brazil, Tanzania, and Madagascar, with the Brazilian sources being the most significant. The only other color changing gemstones are Color-change Garnet and Color-change Sapphire.
Alexandrite is softer than Sapphire and harder then Garnet. The mineral Chrysoberyl The gemstone Chrysoberyl. Have a question about Alexandrite? The Rare Gem LLC is a New York based father-and-son company, with 40 years of experience buying and selling natural alexandrites within the gem trade.
For the first time, they are offering just a few of their most prized V espertine Alexandrites to the public - shop now before they're gone for good. Amazon Supply Store. Clarity Good quality alexandrite has few inclusions. Cut Alexandrite is most often available in mixed cuts.
Carat Weight Most cut gems weigh less than one carat. Larger, higher-quality gems rise in price dramatically. Read Article. Alexandrite Yu S. Russian Alexandrites Karl Schmetzer. Gemstones of the World Walter Schumann. This Month's Featured Gems. Fancy Color Diamond Dazzling brilliance. Emerald Emerald is the bluish green to green variety of beryl, a mineral species that includes aquamarine.
0コメント