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Chrysoberyl
General
Category Mineral
Chemical formula Beryllium aluminium oxide, BeAl2O4
Identification
Color Various shades of green and yellow; brownish, reddish; rarely, blue
Crystal habit slender prisms and tabular form, dimensions are thin in one direction.
Crystal system Orthorhombic 2/m2/m2/m
Twinning Contact and penetration twins common, often repeated forming rosette structures
Cleavage [110] Distinct, [010] Imperfect
Fracture Conchoidal to uneven
Mohs scale hardness 8.5
Luster Vitreous
Streak White
Specific gravity 3.5 - 3.84
Refractive index Biaxial (+) nα=1.745 nβ=1.748 nγ=1.754
Pleochroism Strong in alexandrite
Major varieties
Alexandrite Color change; green to red
Cymophane Chatoyant

The mineral or gemstone chrysoberyl, not to be confused with beryl, is an aluminate of beryllium with the formula BeAl2O4.[1] The name chrysoberyl is derived from the Greek words χρυσός chrysos and βήρυλλος beryllos, meaning "a gold-white spar". Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones. Chrysoberyl is the third-hardest frequently encountered natural gemstone and lies at 8.5 on the hardness scale, between corundum (9) and topaz (8).[2]

An interesting feature of its crystals are the cyclic twins called trillings. These twinned crystals have a hexagonal appearance, but are the result of a triplet of twins with each "twin" oriented at 120o to its neighbors and taking up 120o of the cyclic trilling. If only two of the three possible twin orientations are present, a "V"-shaped twin results.

Ordinary chrysoberyl is yellowish-green and transparent to translucent. When the mineral exhibits good pale green to yellow color and is transparent, then it is used as a gemstone. There are three main varieties of chrysoberyl: ordinary yellow-to-green chrysoberyl, cat's eye or cymophane, and alexandrite. Yellow-green chrysoberyl was referred to as "chrysolite" during the Victorian and Edwardian eras, which caused confusion since that name has also been used for the mineral olivine ("peridot" as a gemstone); that name is no longer used in the gemological nomenclature. Ordinary chrysoberyl is yellowish-green and transparent to translucent. When the mineral exhibits good pale green to yellow color and is transparent, then it is used as a gemstone. Two unusual varieties of chrysoberyl have their own names as gemstones: cat's eye or cymophane, and alexandrite.

Alexandrite, a strongly pleochroic (trichroic) gem, will exhibit emerald green, red and orange-yellow colors depending on viewing direction in partially polarised light. However, its most distinctive property is that it also changes color in artificial (tungsten/halogen) light compared to daylight. The color change from red to green is due to strong absorption of light in a narrow yellow portion of the spectrum, while allowing large bands of blue-greener and red wavelengths to be transmitted. Which of these prevails to give the perceived hue depends on the spectral balance of the illumination. Typically, alexandrite has an emerald-green color in daylight (relatively blue illumination of high color temperature) but exhibits a raspberry-red color in incandescent light (relatively yellow illumination).

Cymophane is popularly known as "cat's eye". This variety exhibits pleasing chatoyant or opalescence that reminds one of an eye of a cat. When cut to produce a cabochon, the mineral forms a light-green specimen with a silky band of light extending across the surface of the stone.

Contents

Occurrence

Chrysoberyl forms as a result of pegmatitic processes. Melting in the Earth's crust produces relatively low-density molten magma which can rise upowards towards the surface. As the main magma body cools, water originally present in low concentrations became more concentrated in the molten rock because it could not be incorporated into the crystallization of solid minerals. The remnant magma thus becomes richer in water, and also in rare elements that similarly do not fit in the crystal structures of major rock-forming minerals. The water extends the temperature range downwards before the magma sets solid completely, allowing concentration of rare elements to proceed so far that they produce their own distinctive minerals. The resulting rock, igneous in appearnce but formed at a low temperature from a water-rich melt, with large crystals of the common minerals such as quartz and feldspar, but also with elevated concentrations of rare elements such as beryllium, lithium, or niobium, often forming their own minerals, is called a pegmatite. The high water content of the magma made it possible for the crystals to grow quickly, so pegmatite crystals are often quite large, which increases the likelihood of gem specimens forming.

Chrsoberyl can also grow in the country rocks near to pergmatites, when Be- and Al-rich fluids from the pegmatite react with surrounding minerals. Hence, it can be found in mica schists and in contact with metamorphic deposits of dolomitic marble. Because it is a hard, dense mineral that is resistant to chemical alteration, it can be weathered out of rocks and deposited in river sands and gravels in alluvial deposits with other gem minerals such as diamond, corundum, topaz, spinel, garnet, and tourmaline. When found in such placers, it will have rounded edges instead of sharp, wedge-shape forms. Much of the chrysoberyl mined in Brazil and Sri Lanka is recovered from placers as the host rocks have been intensely weathered and eroded.

If the pegmatite fluid is rich in beryllium, crystals of beryl or chrysoberyl could form. Beryl has a high ratio of beryllium to aluminium, while the opposite is true for chrysoberyl. Both are stable with the common mineral quartz. For alexandrite to form, some chromium would also have had to be present. However, beryllium and chromium do not tend to occur in the same types of rock. Chromium is commonest in mafic and ultramafic rocks in which beryllium is extremely rare. Beryllium becomes concentrated in felsic pegmatites in which chromium is almost absent. Therefore, the only situation where an alexandrite can grow is when Be-rich pegmatitic fluids react with Cr-rich country rock. This unusual requirement explains the rarity of this chrysoberyl variety.

Chrysoberyl

Yellow chrysoberyl gemstone featuring a brilliant cut crown and step cut pavilion.

Chrysoberyl was discovered in 1789 and described and named by Abraham Gottlob Werner, in 1790. Werner worked at the Freiberg School of Mining from 1790–1793 and was well known as one of the most outstanding geologists of his time. He is best known today as the loser in the battle of the Neptunists and Vulcanists that raged in the 1780s.[3]

Chrysoberyl is normally yellow, yellow-green, or brownish with its color being caused by the presence of iron. Spectroscopic analysis will usually reveal a strong band where the violet takes over from the blue. As the color darkens from bright yellowish-green to golden-yellow to brown, this band increases in strength. When the stone has a strong color, two additional bands can be seen in the green-blue. The most common inclusions are liquid-filled cavities containing three-phase inclusions. Stepped twin planes may be apparent in some cases. Some very rare minty bluish-green chrysoberyls from Tanzania owe their color to the presence of Vanadium.

Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones. Members of the beryl group include emerald, aquamarine, and morganite while members of the chrysoberyl group include chrysoberyl, cymophane (cat's eye), and alexandrite. Beryl is a silicate and chrysoberyl is an oxide and although both beryl and chrysoberyl contain beryllium, they are separate gemstone species unrelated in any other way. Because of the confusion between chrysoberyl and beryl, chrysoberyl is relatively unknown in its own right and the alexandrite variety is much more widely recognized. The only well-known natural gemstones harder than chrysoberyl are corundum and diamond.

Alexandrite

The alexandrite variety displays a color change (alexandrite effect) dependent upon the nature of ambient lighting. This color shift is independent of any change of hue with viewing direction through the crystal that would arise from pleochroism. Both these different properties are frequently referred to as "color change", however. Alexandrite results from small scale replacement of aluminium by chromium ions in the crystal structure, which causes intense absorption of light over a narrow range of wavelengths in the yellow region of the spectrum. Alexandrite from the Ural Mountains in Russia is green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light. The optimum or "ideal" color change would be fine emerald green to fine purplish red, but this is exceedingly rare. Because of their rarity and the color change capability, "ideal" alexandrite gems are some of the most expensive in the world.

According to a widely popular but controversial story, alexandrite was discovered by the Finnish mineralogist Nils Gustaf Nordenskiöld, (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. Sometimes, Nils Gustaf Nordenskiöld is confused with his son, Adolf Erik Nordenskjöld (1832–1901), also a famous Finnish geologist, mineralogist and Arctic explorer who accompanied his father to the Ural Mountains to study the iron and copper mines at Tagilsk in 1853. However, Adolf Erik Nordenskiöld was only two years old when Alexandrite was discovered and only ten years old when a description of the stone was published under the name of Alexandrite for the first time.

Alexandrite step cut cushion, 26.75 cts. Alexandrites this large are extremely rare.

Although it was Nordenskiöld who discovered alexandrite, he could not possibly have discovered and named it on Alexander's birthday. Nordenskiöld's initial discovery occurred as a result of an examination of a newly found mineral sample he had received from Perovskii, which he identified as emerald at first. After the discovery of emeralds in the roots of an upturned tree, the first emerald mine had been opened in 1831, not long before Nordenskiöld had received this particular sample.[4]

Confused with the high hardness however, he decided to continue his examinations. Later that evening, while looking at the specimen under candlelight, he was surprised to see that the color of the stone had changed to raspberry-red instead of green. Later, he confirmed the discovery of a new variety of chrysoberyl, and suggested the name "diaphanite"[2] (from the Greek "di-", twice- and "aphanès", inapparent).

The name of the first person to actually find this stone is unknown. However, the first person to bring it to public attention, and ensure that it would be forever associated with the Imperial family was Count Lev Alekseevich Perovskii (1792-1856.)[5]

The finest alexandrites up to 5 carats (1,000 mg) are being found in the Ural Mountains, but the largest cut stones are in the 30 carats (6.0 g) range, though many fine examples have been discovered in Sri Lanka (up to 65 cts.), India (Andhra Pradesh), Brazil, Myanmar, and especially Zimbabwe (small stones usually under 1 carat (200 mg) but with intense color change). Overall, stones from any locale over 5 carats (1.0 g) would be considered extremely rare, especially gems with fine color change. Alexandrite is both hard and tough, making it very well suited to wear in jewelry.

The gem has given rise to the adjective "alexandritic", meaning any transparent gem or material which shows a noted change in color between natural and incandescent light. Some other gem varieties of which alexandritic specimens have been found include sapphire, garnet, and spinel.

Some gemstones described as lab-grown (synthetic) alexandrite are actually corundum laced with trace elements (e.g., vanadium) or color-change spinel and are not actually chrysoberyl. As a result, they would be more accurately described as simulated alexandrite rather than synthetic but are often called Czochralski Alexandrite after the process that grows the crystals.

Synthetic alexandrite is used as an active laser medium. Alexandrite laser crystals tend to be round, with a pale brown tint.

Genuine alexandrite is one of the most expensive gemstones available commercially, with the stronger color changes being more highly valued. The following are average retail prices for alexandrite in December 2004 from The International Gem Society[6]:

Faceted (Alexandrite) 0.5 carats (100 mg) to 1 carat (200 mg) 1 carat (200 mg) plus
Top Red/Green $5,000 to $15,000/ct to $100,000/ct
Medium Red/Green $3,000 to $9,000/ct to $60,000/ct
Slight Red/Green $100 to $2,500/ct to $6,000/ct
Other colors $1,100 to $8,000/ct to $10,000/ct
Cabochon (Alexandrite) 0.5 carats (100 mg) to 1 carat (200 mg) 1 carat (200 mg) plus
Strong red/green $500 to $2,500/ct to $30,000/ct
Cabochon (Cat's Eye) 0.5 carats (100 mg) to 1 carat (200 mg) 1 carat (200 mg) plus
Strong red/green $1,500 to $5,000 N/A

Cymophane

Fine color Cymophane with a sharp and centered eye.

Translucent yellowish chatoyant chrysoberyl is called cymophane or cat's eye. Cymophane has its derivation also from the Greek words meaning 'wave' and 'appearance', in reference to the chatoyancy sometimes exhibited. In this variety, microscopic tubelike cavities or needlelike inclusions of rutile occur in an orientation parallel to the c-axis producing a chatoyant effect visible as a single ray of light passing across the crystal. This effect is best seen in gemstones cut in cabochon form perpendicular to the c-axis. The color in yellow chrysoberyl is due to Fe3+ impurities.

Although other minerals such as tourmaline, scapolite, corundum, spinel and quartz can form "cat's eye" stones similar in appearance to cymophane, the jewelry industry designates these stones as "quartz cat's eyes", or "ruby cat's eyes" and only chrysoberyl can be referred to as "cat's eye" with no other designation.

Gems lacking the silky inclusions required to produce the cat's eye effect are usually faceted. An alexandrite cat's eye is a chrysoberyl cat's eye that changes color. "Milk and honey" is a term commonly used to describe the color of the best cat's eyes. The effect refers to the sharp milky ray of white light normally crossing the cabochon as a center line along its length and overlying the honey colored background. The honey color is considered to be top-grade by many gemologists but the lemon yellow colors are also popular and attractive. Cat's eye material is found as a small percentage of the overall chrysoberyl production wherever chrysoberyl is found.

Cat's eye really became popular by the end of the 19th century when the Duke of Connaught gave a ring with a cat's eye as an engagement token, this was sufficient to make the stone more popular and increase its value greatly. Until that time, cat's eye had predominantly been present in gem and mineral collections. The increased demand in turn created an intensified search for it in Ceylon. Early 20th century prices could go up as high as $8000 for a cut stone.[7]

See also

References

  1. ^ Chrysoberyl In Webmineral. Retrieved online 08:20, January 25, 2005
  2. ^ a b Klein, Cornelis; and Cornelius S. Hurlbut, Jr. (1985). Manual of Mineralogy (20th ed. ed.). New York: Wiley. ISBN 0-471-80580-7. 
  3. ^ Chapter 3: Species and variety. (2006, February 07). In Alexandrite.net, Tsarstone collectors guide. Retrieved online 06:45, February 26, 2007
  4. ^ Sinkankas, J (1984). Gem Cutting, A Lapidary's Manual. van Nostrom Reinhold. 
  5. ^ Chapter 2: Diaphanite or Alexandrite? (2006, December 07). In Alexandrite.net, Tsarstone collectors guide. Retrieved online 08:20, January 25, 2007
  6. ^ Chapter 9: Alexandrite Buyer's Guide (2006, December 07). In Alexandrite.net, Tsarstone collectors guide. Retrieved online 16:16 PDT, August 9, 2007
  7. ^ "U.S. Geological Survey, 1887 , George Frederick Kunz, Cymophane, Cat's Eye as gemstone". 1887. http://www.farlang.com/gemstones/us-geol-survey-1887/page_031. Retrieved 2007-07-09. 

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

CHRYSOBERYL, a yellow or green gem-stone, remarkable for its hardness, being exceeded in this respect only by the diamond and corundum. The name suggests that it was formerly regarded as a golden variety of beryl; and it is notable that though differing widely from beryl it yet bears some relationship to it inasmuch as it contains the element beryllium. In chrysoberyl, however, the beryllium exists as an aluminate, having the formula BeAl204, or BeO Al 2 O 3. The analysis of a specimen of Brazilian chrysoberyl gave alumina 78.10, beryllia 17.94, and ferric oxide 4.88%. The typical yellow colour of the stone inclines in many cases to pale green, occasionally passing into shades of dark green and brown. The iron usually present in the mineral seems responsible for the green colour. Chrysoberyl is often mistaken by its colour for chrysolite, and has indeed been termed Oriental chrysolite. In its crystalline forms it bears some relationship to chrysolite, both crystallizing in the orthorhombic system, but it is a much harder and a denser mineral. As the two stones are apt to be confounded, it may be convenient to contrast their chief characters: - Chrysoberyl. Chrysolite. Hardness. ... 8.5 6.5 to 7 Specific Gravity.. 3.65 to 3.75 3.34 to 3.37 Chemical Composition.. BeAl 2 O 4. Mg2S104.

Chrysoberyl is not infrequently cloudy, opalescent and chatoyant, and is then known as "cymophane" (Gr. Kuµa, a "cloud"). The cloudiness is referable to the presence of multitudes of microscopic cavities. Some of the cymophane, when cut with a convex surface, forms the most valuable kind of cat's-eye (see CAT'S-EYE). A remarkable dichroic variety of chrysoberyl is known as alexandrite.

Most chrysoberyl comes from Brazil, chiefly from the district of Minas Novas in the state of Minas Geraes, where it occurs as small water-worn pebbles. The cymophane is mostly from the gem-gravels of Ceylon. Chrysoberyl is known as a constituent of certain kinds of granite, pegmatite and gneiss. In the United States it occurs at Haddam, Conn.; Greenfield Centre, near Saratoga Springs, N.Y.; and in Manhattan island. It is known also in the province of Quebec, Canada, and has been found near Gwelo in Rhodesia. (F. W. R.*)


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