Tuesday, 12 February 2013

Introduction to Chrysoberyl

Rough tabular "trilling" (three intergrown crystals) of alexandrite chrysoberyl from the Urals.Crystal system Orthorhombic


Appearance Single crystals are rare. Chrysoberyl is usually found as V-shaped twins or sometimes re twinning of flattened crystals, giving an appearance of hexagonal symmetry. It is normally cloudy, but when transparent, has considerable luster. The most common color yellow, but it can also be grayish, greenish yellow, or more or less green.

Physical properties


It is quite hard: 8.5 on Mohs scale with a density of around 3.7 g/cm3. The refractive indices are roughly n  1.74 and n  1.75. It has weak pr' cleavage.

Genesis


Chrysoberyl occurs in granite pegmatiles and is an uncommon mineral. Occurrence It is found in some pegmatites in the Un States, in the Ural mountains in the Soviet Union, and in Sri Lanka, Rhodesia, Brazil, Madagascar, and Italy.

Alexandrite


This extremely rare gemstone of fairly recent history its name to the fact that it was first discovered in the in 1830, on the day of Prince Alexander of Russia's of age.

Appearance


Alexandrite, 0.15 ct, Zimbabwe.Green: it can be almost emerald-colored, but is more often yellowish or brownish green. Its c characteristic is the ability to change color if exposed light source rich in red rays: by candlelight or lungs' light, it turns red or reddish. This unusual phenomenal; what distinguishes it from other green chrysoberyl pronounced color change from green to red is highly prized and, as always, the exact tone of the color, or colors, is portant, the ideal being brilliant green turning to fiery although dull green turning reddish or slightly turbid blood red is more common, and given alexandrite's extreme rarity, even stones with a number of inclusions are cut. It given an oval or round, mixed cut, or a rectangular, cut. Alexandrite is so rare that few people have ever one: and perhaps for this reason, the name is applied other, mainly synthetic stones, not remotely like ills violet-colored synthetic corundums).

Distinctive features


The changing color is an unmistakable characteristic. However, in the last few years, quantities of green or bluish-colored grossular garnet t turn red like alexandrite have been found in East Africa. To distinguish between the two, it is useful to look for signs birefringence, as garnet, unlike alexandrite, is singly refractive. The refractive indices of the two stones, on the other hand, are very similar, as are their densities.

Occurrence


Found in limited quantities in the So Union, Brazil, Sri Lanka, and some East African count (Zimbabwe and Tanzania).

Value


Twinned crystal of true chrysoberyl from Brazil.Fine stones are extremely valuable and on a with emeralds, rubies, and sapphires. When the colors dullish, the value falls appreciably, but because of its rarity all alexandrite is expensive. It is a collector's item.

Simulants and synthetics


Attempts have been rat the past to imitate alexandrite's change in color using synthetic spinet and corundum, but the results have modest. More recently, a violet-colored synthetic has been widely marketed under the name of despite its lack of any real similarity to the natural stone However, many who have never seen true alexandrite apply the name to this synthetic corundum and possibly for this reason, some manufacturers of synthetic cons call their violet stones "alexandrite type." Synthetic alexandrite has also recently been produced, but mainly for industrial purposes. For the time being, therefore, set alexandrite is even rarer than the natural variety.

Chrysoberyl


This is the true chrysoberyl, also known as golden chrysoberyl, a synonym, the prefix "chryso" being Grew "golden." It is the most common variety of chrysoberyl

Appearance


The color varies from yellow to yellow or green, brownish yellow, or pale yellow. It has basically the same color range as cat's eye (cymophane) with a bit more green or brown.

Two cabochons of cymophane, the cat's-eye par excellence.
Mixed oval or round cuts or even triangular, marquise, or pear-shaped cuts are all used any cut, in fact, which can set off the excellent luster of the stone, and allow it to be used even in jewelry of complex design.

Distinctive features


The colors can frequently N. similar to that of certain beryls, or to some practically yellow olivines; but it has greater luster than these two It also has quite different refractive indices and ac density from that of beryl. When the color is pale can closely resemble what is known as yellow saw but the density is different.

Occurrence


Chrysoberyl is mainly found in Brut, Lanka, and Madagascar.

Value


Although of some importance in the past, it comparatively little known and appreciated today, pro account of its rather weak color. Its value is acc quite low, about the same as that of beryls, tour and spinets of unexceptional color.

Simulants and synthetics


Very, occasionally a greenish yellow synthetic spinets. which could be regarded as imitations of chrysoberyl, appear on the gem mul4 Synthetic yellow chrysoberyl, on the other hand, does not appear to exist on the market, which is hardly surprising given the modest value of the genuine article and the complexity and high cost of synthesis.

Cat's-eye or Cymophane


This is the name given to the yellow, yellow-green or gray-green variety of chrysoberyl, which displays the phenomenon of chatoyancy because of the inclusion of numerous fine, parallel crystal needles.

Appearance



 Chrysoberyl.
The color is greenish yellow or yellowish sometimes with a rather cold, almost grayish tone. Some fine stones are a honey brown. In the proper light, the near side will appear yellowish white while the brown of the far side will be intensified; creating a milk-and-honey effect Cat's-eye is always cut en cabochon. round or oval, to emphasize the cat's eye effect, and can be fairly transparent:, Due to its hardness, it takes and maintains good luster the more pronounced and pleasing the cat's-eye effect the greater is its value.

Distinctive features


The most common cat's-eye stone are the quartzes, which, however, usually have a rather different color from chrysoberyl and are less transparently brighter, but more superficial, chatoyancy. If there Isere doubt, they can immediately be distinguished by their different density, because refractive indices are always to establish for curved stones.

Occurrence


Cymophanes are mainly found in Sri and Brazil, although they are not common.

Value

Two cabochons of cymophane, the cat's-eye par excellence.

Cat's-eye is highly prized by collectors and connoisseurs. Its value is accordingly quite high. Very fine emotes are less valuable than the principal gemstone including alexandrite, but more so, for example, than a fine topaz or spinel. Due to its value and hardness, it is also known as -noble cat's-eye.-

Simulants and synthetics


Various natural stones been used as substitutes, including fluorite, which, have ever, is much softer, and kornerupine, a gem still rather than cymophane, with lower hardness and density. Duarte to also been used, although its color rarely resembles theta cymophane and its density is also much lower. To knowledge, no attempts have been made to produce this gem synthetically.

Writer – Curzio Cipriani & Alessandro Borelli
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