Alexandrite

Rare Alexandrite Gems

Alexandrite Rough from Tanzania

Alexandrite is one of the rarest of all colored gemstones and is famed for its color change from green in daylight to red under incandescent light. Fine faceted alexandrite over 1 carat is more valuable than sapphire, ruby or emerald.

Alexandrite is a strongly pleochroic gem that will exhibit emerald green, red and orange-yellow colors and tend to change color in artificial light compared to daylight. The color change from red to green is due to strong absorption of light in the yellow and blue portions of the spectrum. Typically, alexandrite has an emerald-green color in daylight but exhibits a raspberry-red color in incandescent light.

According to a popular but probably apocryphal story, alexandrite was discovered by the Finnish mineralogist Nils Gustaf Nordenskjold, (1792 -1866) on the tsarevitch Alexander's sixteenth birthday on April 17, 1834 and named alexandrite in honor of the future Tsar Alexander II of Russia. It is apparently true that alexandrite was first identified (but not discovered) by Nordenskjold, but likely some years earlier, around 1831. It was Count Lev Alekseevich Perovskii (1792-1856) who named the gem alexandrite.

Tsar Alexander the II

Alexandrite is a variety of chrysoberyl, with an excellent hardness of 8.5 on the Mohs scale. Chyrsoberyl has a refractive index of 1.746-1.763 and a density or specific gravity of 3.70-3.78.

Alexandrite is very rare because of its chemical composition. While it is a form of chrysoberyl, it has an extra trace elemement in addition to iron and titatnium. It is the presence of chromium that gives it the emerald-green hue in daylight. Alexandrite exhbiting chatoyancy or the cat's eye effect is known, but very rare.

The original source of alexandrite in the Ural mountains in Russia has been worked out. An important alexandrite deposit was found in 1987 in Minas Gerais in Brazil, and other deposits are found in Burma, Madagascar, Sri Lanka and Tanzania.

Alexandrite is rarely found in larger sizes. The largest cut alexandrite weighs 66 carats and is in the Smithsonian Institution in Washington, D.C.

Introduction

Chrysoberyl has been known since ancient times and the name comes from the Greek word for gold. Its most famous varieties include alexandrite, named after Czar Alexander II, and chrysoberyl cat's eye, also called cymophane. Chyrsoberyl is a very hard gem, rating 8.5 on the Mohs scale right behind corundum and diamond.

Chrysoberyl colors

Transparent faceted Chrysoberyl is usually found in golden yellow, yellowish-green to green, yellow, as well as shades of brown and red. Alexandrite, the rare color-change variety, is green in daylight and reddish in artificial incandescent light.

Buying Chrysoberyl

Color

As for most gemstones, value tends to vary with color saturation.

Lighting

Alexandrite is green in daylight and light red in artificial incandescent light.

Clarity

Chrysoberyl usually has excellent brilliance and luster. Eye clean stones should be the first choice.

The cat's eye effect is caused by microscopic needle-like inclusions. The term "cat's eye" always refers to chrysoberyl cat's eyes whereas all other cat's eyes must be designated by an additional name, like "tourmaline cat's eye".

Cut

Stones are fashioned mainly in step and brilliant cut. Cat's eyes are fashioned in cabochon cut.

Chrysoberyl location and deposits

Chrysoberyl deposits are in Brazil (Minas Gerais, Esperito Santo), Madagascar, Myanmar (Burma), Russia, United States and Zimbabwe.

Chysoberyl cat's eyes are found in Brazil, China, India, Sri Lanka and Zimbabwe.

Alexandrite deposits are in Brazil (Minas Gerais), Madagascar, Myanmar (Burma), Sri Lanka and Tanzania.

Common Chrysoberyl treatments

None

World-famous Chrysoberyl

The famous "Hope" chrysoberyl in London, a light green, faceted stone of 45 ct is completely clean.

The largest alexandrite, with a weight of 1876 ct, was found in Sri Lanka.

The largest cut alexandrite, 66 ct, belongs to the Smithsonian Institution in Washington D.C.

Chrysoberyl gemology

Species: Beryl

Color: Golden-yellow, green-yellow, green, brownish, red

Chemical composition: BeAl2O4 beryllium

Crystal system: (Orthorhombic) thick-tabled, intergrown triplets

Hardness: 8.5 (Mohs scale)

Specific gravity: 3.70 - 3.78

Refractive index: 1.746 - 1.763

Birefringence: +0.007 to +0.011

Optical character:

Color of streak: White

Absorption spectrum: 504, 495,485, 445

Fluorescence: Usually none. Green: weak; dark red

The Chrysoberyl zodiac, myth & legend

Chrysoberyl and its varieties did not make it into the books of mythical and legendary fame.

http://www.gemselect.com