Thursday, 21 February 2013

Introduction to Beryl Gemstone

Crystal of emerald berylSilicate of beryllium and aluminum

Crystal system


Hexagonal

Appearance


Beryl crystallizes as fairly complete hexagonal prisms, sometimes with basal faces or small bipyramidal facets. The crystals can be very large from a few centimeters, up to some tens of centimeters with occasional specimens over a meter in length. Beryl is often cloudy and, when transparent, has a vitreous luster. It is usually an opaque, milky white, or a faint yellow, very pale gray or green color. Stronger, more attractive colors, how-ever, also occur: mainly blue, green, yellow or pink. Al-though rare, red and colorless specimens do exist. Crystals that combine brilliant color with transparency are highly prized as gems.

Physical properties


It has a hardness of 7.5 to 8, but is fairly brittle, and may show ill-defined cleavage parallel to the basal plane. The density is normally 2.67-2.72 gicre, but can be as much as 2.90 g/cm3. The refractive indices, like the density, are somewhat variable, from or 1.560. n1.57010 or 1.595, nu: 1.602. Genesis Beryl occurs as an accessory mineral in granite rocks, and crystallizes mainly in pegmatites, where the largest individual crystals are found. It is also formed by metasomatism in the country rock surrounding pegmatites, and is associated with hydrothermal processes. Occurrence Beryl is very widespread; it occurs in Pegmatites in many areas from northern Europe to North America, South America, East Africa, South Africa, and Himalayan Asia. It occurs also in calcite veins, as a process of hydrothermal activity in the bituminous limestones of Bogota, Colombia.

Emerald


Be3 (Al, Cr)2 Si6018 The name is of ancient origin. The Latin smaragdus appears, in fact, to have referred to the stone we call emerald, which is now considered as a distinct species. II is basically the green variety of beryl, although not all gem-quality green beryls are called emeralds: yellow-green stones are called heliodors; soft blue-green or even pale green specimens (their color due to iron, not chromium, as in emerald) are called aquamarines.

Appearance

Inclusions in an emerald

The typical color is a beautiful, distinctive hue known, in fact, as emerald green and is due to traces 1 of chromium in the crystal structure. But emeralds can be light or dark green, bright green or leaf green. The vitreous luster is not outstanding, and is strongest in medium light stones with few inclusions. All emerald contains inclusions, although in the best quality stones, these are very faint and not visible to the naked eye. They show up under a 10X, 20x or 40x lens. The most common shape for gems is the step or trap cut, which also known as the emerald is cut. They are occasionally given a mixed, oval cut, while antique stones are found with hexagonal, step cuts, cabochon cuts, or pear shapes with a hole in them, often used as pendants.

Distinctive features


The typical emerald color is virtually unmistakable. It is only equalled by some very rare specimens of jadeite jade, which, however, is less transparent and has different physical properties. To the initiated, the inclusions in emerald can be highly distinctive: a bubble at gas in a liquid (like a spirit level), within spindle-shaped or, more rarely, truncated prismatic cavities; birefringent, circular plates of mica; multifaceted pyrite crystals or calcite rhombohedra. 

However, a microscope is almost always needed to recognize them. Although not the typical emerald color, some green tourmalines may took similar, but they can be distinguished either by their marked pleochroism, or by the fact that tourmalines which are given an emerald cut display alternating, longitudinal lines of lighter or darker color, when viewed through the table facet. Olivine may also be a verdant green color vaguely similar to that of some atypical emeralds; but the powerful birefringence of olivine is detectable with a simple lens, a double image of the opposite facet edges being clearly visible in certain directions through the table facet. In any case, the density of either tourmaline or olivine immediately distinguishes the stone from emerald.

Occurrence


 Introduction to Beryl GemstoneThe biggest and most beautiful emeralds come from the famous Chivor and Muzo mines of Colombia. Much smaller quantities of emeralds, mostly of me dium-light color, come from Brazil, and small, very intensely colored stones, characterized by numerous minute inclusions of molybdenite with a metallic appearance, are found in the Transvaal.


In the last few decades, increasing quantities of emeralds have been found in a series of small deposits in East Africa principally in Zimbabwe, Zambia and Tanzania. These are quite a strong color, sometimes with a bluish, green tinge; and they often contain mica plates and, some-times, thin crystal needles. The most famous of these emeralds are the ones from Sandawana in Zimbabwe, which are valued for their color. Emeralds with similar characteristics also come from the mountains of India and Pakistan, as well as the Soviet Union (Urals), and formerly Austria

Value


Stones of fine color, weighing more than 2 carats are among the most highly valued gemstones, and then price may equal or exceed that of diamonds. Less idea: colored varieties too dark or too pale are worth a lot less; and if they are slightly turbid as well, the value is reduced even further.

Simulants and synthetics


The Romans are known to have imitated emerald with skillfully worked green glass. Glass was also used in later centuries, extraneous particles sometimes being incorporated to simulate incisions.
 Introduction to Beryl Gemstone

Doublets have also been used as imitations, with a lower portion of green glass and a top portion of garnet, or triplets, with a layer of colored cement sandwiched between two layers of colorless beryl, synthetic spinet, or quad! Synthetic emeralds have likewise been widely produced over the last few decades. Generally of good color, these are mainly distinguished from the natural variety by her inclusions and other growth features. There are a lot of these synthetic stones about, but their cost is quite high, so that the market for them is saturated.

6.2 Aquamarine


The name refers to the palish blue, light blue-green or even light green variety of beryl. The green of aquamarine is a watery green without any trace of yellow and is due to iron, not chromium, as can be seen from examination with a gemological spectroscope.

Appearance


The most valuable color is a rich, sky blue: but because the stone is pleochroic, even the blue stones have a green or greenish-blue tinge in one direction. Quite large stones, ranging from several carats to more than ten or a few tens of carats, are relatively common. Many are virtually free of inclusions. (Again, where there is plenty of material available, poor quality specimens do not usually come to market.) The luster is vitreous and not exceptional. The most common cut is the emerald type, although mixed oval or pear-shaped cuts are not infrequent.

Distinctive features


 Introduction to Beryl Gemstone
The color of this stone, combined with its particular type of pleochroism and vitreous luster, distinguishes it fairly easily from blue topaz and light-blue synthetic spinet, the first being a definite blue color the second having a gray or violet tinge, much stronger luster and no pleochroism.

Occurrence


Most aquamarine comes from the pegmatites of Brazil, where crystals weighing several kilos have been found. Other deposits are in the Soviet Union (Trans-baikalia, Urals, and Siberia), Madagascar, the United States, and, recently, Afghanistan. Value Rich blue stones several carats in weight are among the most valuable of secondary gems. They are worth a lot more, for instance, than blue topaz of similar characteristics. Pale or green stones are much less valuable. Simulants and synthetics Aquamarine is imitated by blue glass, which faithfully reproduces the color, if not the pleochroism, but it is most often imitated by blue synthetic spinet, of a slightly different color, with superior luster and no pleochroism. 

Because of the general similarity, this is sometimes called synthetic aquamarine, although the latter, as such, is not produced. Light green or yellow-green beryl can be turned blue by heating it to a certain temperature for a certain length of time. This practice has been in use for several decades and is considered acceptable, as with zircon and sapphire.

Writer – Curzio Cipriani and Alessandro Borelli
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