Alexandrite

Alexandrite
Alexandrite is a Rare Variety of ChrysoberylAlexandrite was discovered near Ekaterinburg in the Ural Mountains in Russia in 1830, and was named after Alexander II. Other sources now include Sri Lanka, Burma, Brazil, Zimbabwe, Madagascar and North America.The important feature of Alexandrite which makes it highly unusual and collectable amongst gemstones is its strong colour change when viewed in different lights. Many other stones show a similar property, but nowhere as strongly, and not with such highly contrasting colours.It is hardly ever seen in the U.K. because of its rarity and high price.
Colour:Chrysoberyl is mainly green, yellow or brown. High quality alexandrite varies from grass green to raspberry red, although some stones will tend towards olive green or brown, and others to yellow. The red and green colouration is caused by the partial replacement of aluminium by chromium.In daylight, stones appear green, whilst in tungsten lighting they will change colour to red. This effect is caused by a combination of factors. Alexandrite is bi-refringent (doubly refractive), and bi-axial, so that it possesses three different refractive indexes in its three different optical directions. Each of these three directions has a strongly different absorption spectrum, causing different colours to be seen when viewed from different directions in relation to the crystal structure, an effect known as pleochroism. Daylight contains high proportions of blue light, tungsten lighting contains a higher balance of red light. The two main optical directions of alexandrite absorb light of different wavelengths in considerably different ways. If this sounds technical and complicated, it is because it is technical and complicated. I will try to return to this paragraph later when I have worked out a simpler sounding way to explain the phenomenon. Any suggestions welcome.