domingo, 26 de maio de 2013

Emerald

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For other uses, see Emerald (disambiguation).

Emerald
Emerald crystal from Muzo, Colombia
General
Category	Beryl variety
Formula (repeating unit)	Be₃Al₂(SiO₃)₆
Crystal symmetry	(6/m 2/m 2/m) – Dihexagonal Dipyramidal
Unit cell	a = 9.21 Å, c = 9.19 Å; Z = 2
Identification
Formula mass	537.50
Color	Green shades to colorless
Crystal habit	Massive to well Crystalline
Crystal system	Hexagonal (6/m 2/m 2/m) Space group: P6/mсc
Cleavage	Imperfect on the [0001]
Fracture	Conchoidal
Mohs scale hardness	7.5–8
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to opaque
Specific gravity	Average 2.76
Optical properties	Uniaxial (-)
Refractive index	n_ω = 1.564–1.595, n_ε = 1.568–1.602
Birefringence	δ = 0.0040–0.0070
Ultraviolet fluorescence	None (some fracture filling materials used to improve emerald's clarity do fluoresce, but the stone itself does not)
References	^[1]

Emerald is a gemstone, and a variety of the mineral beryl (Be₃Al₂(SiO₃)₆) colored green by trace amounts of chromium and sometimes vanadium.^[2] Beryl has a hardness of 7.5–8 on the 10-point Mohs scale of mineral hardness.^[2] Most emeralds are highly included, so their toughness (resistance to breakage) is classified as generally poor.

Etymology

The word "Emerald" is derived (via Old French: Esmeraude and Middle English: Emeraude), from Vulgar Latin: Esmaralda/Esmaraldus, a variant of Latin Smaragdus, which originated in Greek: σμάραγδος (smaragdos; "green gem").

Properties determining value

Cut emeralds

Emeralds, like all colored gemstones, are graded using four basic parameters–the four Cs of Connoisseurship: Color, Cut, Clarity and Crystal. The last C, crystal, is simply a synonym for transparency, or what gemologists call diaphaneity. Before the 20th century, jewelers used the term water, as in "a gem of the finest water,"^[3] to express the combination of two qualities: color and crystal. Normally, in the grading of colored gemstones, color is by far the most important criterion. However, in the grading of emeralds, crystal is considered a close second. Both are necessary conditions. A fine emerald must possess not only a pure verdant green hue as described below, but also a high degree of transparency to be considered a top gem.^[4]
In the 1960s, the American jewelry industry changed the definition of 'emerald' to include the green vanadium-bearing beryl as emerald. As a result, vanadium emeralds purchased as emeralds in the United States are not recognized as such in the UK and Europe. In America, the distinction between traditional emeralds and the new vanadium kind is often reflected in the use of terms such as 'Colombian Emerald.'^[5]

Color

Scientifically speaking, color is divided into three components: hue, saturation and tone. Emeralds occur in hues ranging from yellow-green to blue-green, with the primary hue necessarily being green. Yellow and blue are the normal secondary hues found in emeralds. Only gems that are medium to dark in tone are considered emerald; light-toned gems are known instead by the species name green beryl. The finest emerald are approximately 75% tone on a scale where 0% tone would be colorless and 100% would be opaque black. In addition, a fine stone should be well saturated; the hue of an emerald should be bright (vivid). Gray is the normal saturation modifier or mask found in emerald; a grayish-green hue is a dull green hue.^[4]
Emeralds are green by definition (the name is derived from the Greek word 'smaragdus', meaning green).^[6] Emeralds are the green variety of beryl, a mineral which comes in many other colors that are sometimes also used as gems, such as blue aquamarine, yellow heliodor, pink morganite and colorless goshenite.^[7]

Clarity

Emerald tends to have numerous inclusions and surface breaking fissures. Unlike diamond, where the loupe standard, i.e. 10X magnification, is used to grade clarity, emerald is graded by eye. Thus, if an emerald has no visible inclusions to the eye (assuming normal visual acuity) it is considered flawless. Stones that lack surface breaking fissures are extremely rare and therefore almost all emeralds are treated, "oiled", to enhance the apparent clarity. Eye-clean stones of a vivid primary green hue (as described above) with no more than 15% of any secondary hue or combination (either blue or yellow) of a medium-dark tone command the highest prices.^[4] This relative crystal non-uniformity makes emeralds more likely than other gemstones to be cut into cabochons, rather than faceted shapes.

Treatments

Most emeralds are oiled as part of the post-lapidary process, in order to improve their clarity. Cedar oil, having a similar refractive index, is often used in this generally accepted practice. Other liquids, including synthetic oils and polymers with refractive indexes close to that of emerald such as Opticon are also used. The U.S. Federal Trade Commission requires the disclosure of this treatment when a treated emerald is sold.^[8] The use of oil is traditional and largely accepted by the gem trade. Other treatments, for example the use of green-tinted oil, are not acceptable in the trade. The laboratory community has recently standardized the language for grading the clarity of emeralds. Gems are graded on a four step scale; none, minor, moderate and highly enhanced. Note that these categories reflect levels of enhancement, not clarity. A gem graded none on the enhancement scale may still exhibit visible inclusions. Laboratories tend to apply these criteria differently. Some gem labs consider the mere presence of oil or polymers to constitute enhancement. Others may ignore traces of oil if the presence of the material does not materially improve the look of the gemstone.
Given that the vast majority of all emeralds are treated as described above, and the fact that two stones that appear visually similar may actually be quite far apart in treatment level and therefore in value, a consumer considering a purchase of an expensive emerald is well advised to insist upon a treatment report from a reputable gemological laboratory. All other factors being equal, a high quality emerald with moderate enhancement should cost half the price of an identical stone graded none.^[4]

Emerald localities

Spanish emerald and gold pendant exhibited at Victoria and Albert Museum.

Emeralds in antiquity^[when?] were mined in Egypt, India, and Austria.^[9]
Colombia is by far the world's largest producer of emeralds, constituting 50–95% of the world production, with the number depending on the year, source and grade.^[10]^[11]^[12]^[13] Emerald production in Colombia has increased drastically in the last decade, increasing by 78% from 2000 to 2010.^[14] The three main emerald mining areas in Colombia are Muzo, Coscuez, and Chivor.^[15] Rare 'trapiche' emeralds are found in Colombia, distinguished by a six-pointed radial pattern made of ray-like spokes of dark carbon impurities.^{[citation needed]}
Zambia is the world's second biggest producer, with its Kafubu River area deposits (Kagem Mines) about 45 km southwest of Kitwe responsible for 20% of the world's production of gem quality stones in 2004.^[16] In the first half of 2011 the Kagem mines produced 3.74 tons of emeralds.^[17] Zambian emeralds are of very high quality, being less porous and brittle than Colombian emeralds, with more even color.
Emeralds are found all over the world in countries such as Afghanistan, Australia, Austria, Brazil,^[18] Bulgaria, Cambodia, Canada, China, Egypt, Ethiopia, France, Germany, India, Italy, Kazakhstan, Madagascar, Mozambique, Namibia, Nigeria, Norway, Pakistan, Russia, Somalia, South Africa, Spain, Switzerland, Tanzania, United States, Zambia, and Zimbabwe.^[1] In the US, emeralds have been found in Connecticut, Montana, Nevada, North Carolina, and South Carolina.^[1] In 1997 emeralds were discovered in the Yukon.^[19]

Synthetic emerald

Emerald showing its hexagonal structure

Both hydrothermal and flux-growth synthetics have been produced, and a method has been developed for producing an emerald overgrowth on colorless beryl. The first commercially successful emerald synthesis process was that of Carroll Chatham, likely involving a lithium vanadate flux process, as Chatham's emeralds do not have any water and contain traces of vanadate, molybdenum and vanadium.^{[verification needed]} The other large producer of flux emeralds was Pierre Gilson Sr., whose products have been on the market since 1964. Gilson's emeralds are usually grown on natural colorless beryl seeds, which are coated on both sides. Growth occurs at the rate of 1 mm per month, a typical seven-month growth run producing emerald crystals of 7 mm of thickness.^[20] Gilson sold his production laboratory to a Japanese firm in the 1980s, but production has since ceased; so has Chatham's, after the 1989 San Francisco earthquake.
Hydrothermal synthetic emeralds have been attributed to IG Farben, Nacken, Tairus, and others, but the first satisfactory commercial product was that of Johann Lechleitner of Innsbruck, Austria, which appeared on the market in the 1960s. These stones were initially sold under the names "Emerita" and "Symeralds", and they were grown as a thin layer of emerald on top of natural colorless beryl stones. Although not much is known about the original process, it is assumed that Leichleitner emeralds were grown in acid conditions.^{[citation needed]} Later, from 1965 to 1970, the Linde Division of Union Carbide produced completely synthetic emeralds by hydrothermal synthesis. According to their patents,^[21]^[22] acidic conditions are essential to prevent the chromium (which is used as the colorant) from precipitating. Also, it is important that the silicon-containing nutrient be kept away from the other ingredients to prevent nucleation and confine growth to the seed crystals. Growth occurs by a diffusion-reaction process, assisted by convection. The largest producer of hydrothermal emeralds today is Tairus in Russia, which has succeeded in synthesizing emeralds with chemical composition similar to emeralds in alkaline deposits in Colombia, and whose products are thus known as “Colombian Created Emeralds” or “Tairus Created Emeralds”.^[23] Luminescence in ultraviolet light is considered a supplementary test when making a natural vs. synthetic determination, as many, but not all, natural emeralds are inert to ultraviolet light. Many synthetics are also UV inert.^[24]
Synthetic emeralds are often referred to as "created", as their chemical and gemological composition is the same as their natural counterparts. The U.S. Federal Trade Commission (FTC) has very strict regulations as to what can and what cannot be called "synthetic" stone. The FTC says: "§ 23.23(c) It is unfair or deceptive to use the word "laboratory-grown," "laboratory-created," "[manufacturer name]-created," or "synthetic" with the name of any natural stone to describe any industry product unless such industry product has essentially the same optical, physical, and chemical properties as the stone named."^[25]

Emerald in different cultures, and emerald lore

The Gachala Emerald is one of the largest gem emeralds in the world, at 858 carats (172 g). This stone was found in 1967 at La Vega de San Juan mine in Gachalá, Colombia. It is housed at the National Museum of Natural History of the Smithsonian Institution in Washington, D.C.

Emerald is regarded as the traditional birthstone for May, as well as the traditional gemstone for the astrological signs of Taurus, Gemini and sometimes Cancer. One of the quainter anecdotes on emeralds was by the 16th-century historian Brantôme, who referred to the many impressive emeralds the Spanish under Cortez had brought back to Europe from Latin America. On one of Cortez's most notable emeralds he had the text engraved Inter Natos Mulierum non sur-rexit mayor ("Among those born of woman there hath not arisen a greater," Matthew 11:11) which referred to John the Baptist. Brantôme considered engraving such a beautiful and simple product of nature sacrilegious and considered this act the cause for Cortez's loss of an extremely precious pearl (to which he dedicated a work, A beautiful and incomparable pearl), and even for the death of King Charles IX of France, who died soon after.^[26]

Notable emeralds

Emerald	Origin
Chalk Emerald	Colombia
Duke of Devonshire Emerald
Gachala Emerald
Mogul Mughal Emerald
Bahia Emerald	Brazil

Gallery

Left to right: The Indian Emerald Necklace, the Gachala Emerald crystal and the Mackay Emerald Necklace (167-carat center stone), all pieces from the U.S. National Museum of Natural History.
The Chalk Emerald ring, containing a top-quality 37-carat emerald, also in the U.S. National Museum of Natural History.
The Hooker Emerald Brooch, containing a 75-carat square-cut emerald, also in the U.S. National Museum of Natural History.
Emerald crystal (about 1 cm) in calcite matrix, Muzo, Colombia.
A 5-carat emerald

Chrysoberyl

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Chrysoberyl

General
Category	Oxide minerals - Spinel group
Formula (repeating unit)	BeAl₂O₄
Strunz classification	04.BA.05
Crystal symmetry	Orthorhombic 2/m2/m2/m dipyramidal
Unit cell	a = 5.481 Å, b = 9.415 Å, c = 4.428 Å; Z = 8
Identification
Color	Various shades of green, yellow, brownish to greenish black, may be raspberry-red under incandescent light when chromian; colorless, pale shades of yellow, green, or red in transmitted light
Crystal habit	Crystals tabular or short prismatic, prominently striated
Crystal system	Orthorhombic
Twinning	Contact and penetration twins common, often repeated forming rosette structures
Cleavage	Distinct on (110), imperfect on (010), poor on {001}
Fracture	Conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	8.5
Luster	Vitreous
Streak	White
Specific gravity	3.5 - 3.84
Optical properties	Biaxial (+)
Refractive index	n_α=1.745 n_β=1.748 n_γ=1.754
Pleochroism	X = red; Y = yellow-orange; Z = emerald-green
2V angle	Measured: 70°
References	^[1]^[2]^[3]
Major varieties
Alexandrite	Color change; green to red
Cymophane	Chatoyant

The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula Be Al₂O₄.^[3] 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).^[4]
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 120° to its neighbors and taking up 120° 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. The three main varieties of chrysoberyl are: 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.
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 chatoyancy 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.

Occurrence

Chrysoberyl forms as a result of pegmatitic processes. Melting in the Earth's crust produces relatively low-density molten magma which can rise upwards 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 appearance 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.
Chrysoberyl can also grow in the country rocks near to pegmatites, 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.

Alexandrite

The alexandrite variety displays a color change (alexandrite effect) dependent upon the nature of ambient lighting. Alexandrite effect is the phenomenon of an observed color change from greenish to reddish with a change in source illumination due physiological response of the human eye in a particular part of the visible spectrum. This color change is independent of any change of hue with viewing direction through the crystal that would arise from pleochroism. 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.^{[citation needed]}
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.

Alexandrite step cut cushion, 26.75 cts.

According to a popular but controversial story, alexandrite was discovered by the Finnish mineralogist Nils Gustaf Nordenskiöld (1792–1866), and named alexandrite in honor of the future Tsar Alexander II of Russia. 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. ^[5] The first emerald mine had been opened in 1831.
Alexandrite up to 5 carats (1,000 mg) and larger were traditionally thought to be found only in the Ural Mountains, but have since been found in larger sizes in Brazil. Other deposits are located in India (Andhra Pradesh), Madagascar, and Sri Lanka. Alexandrite in sizes over three carats are very rare.
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. ^[6]

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 ^[7] 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 Sri Lanka. Early 20th century prices could go up as high as $8000 for a cut stone.^[8]

quarta-feira, 22 de maio de 2013

10 pedras mais conhecidas

Diamante

O diamante é, das pedras preciosas, _dsc2961_morguefile

a mais valiosa e resistente. Seu significado de nome origina-se do grego adamas, que significa inconquistável. Esse mineral é uma forma de carbono, assim como o grafite e o carvão. Ele se cristaliza no sistema cúbico, à pressões e temperaturas extremamente elevadas, ou seja, ele é carbono cristalizado.

É conhecido há mais de 3000 anos, quando foi descoberto na Índia. No Brasil, foi a primeira pedra a ser comercializada, quando foi encontrada em Diamantina, no ano de 1725. Nessa época, o Brasil se tornou o maior comercializador de diamantes do mundo, seguido da África do Sul, que logo depois nos ultrapassou com a descoberta de milhares de depósitos da pedra preciosa. Por seu grande valor, ele é a pedra preciosa mais explorada e estudada pelos homens.

Suas cores podem variar de transparente a azul, verde, rosa e raramente vermelho, podendo também ser amarelados ou castanhos. Desses tipos, os mais valiosos são os incolores e os de cores fortes ou raras. Estudos mostram que o diamante se forma na parte interna da terra, em profundidades de 100 a 200 km.

O diamante é tornado menos grosso e polido com pó de diamante, por causa de sua fragilidade. Uma característica muito interessante dessa gema é a tendência de aparecimento de uma luz fluorescente azul-baça, quando exposto a raio X. Essa técnica é utilizada para separar o diamante de sua rocha hospedeira, bruta.

Composição: Carbono

Fórmula química: C

Opala

A opala divide-se em duas classificações: a opala preciosa e a opala comum. Seu nome deriva-se de upala, que significa 'pedra preciosa'.

Sua principal característica são as inúmeras colorações fundidas, que formam um jogo de cores belíssimo. A opala é uma das poucas pedras preciosas que não é cristalina. As cores da preciosa variam: na opala branca as cores, são mais pálidas, claras, e nas opalas escuras, as cores são castanhas, cinzentas e negras. Na opala de fogo, as cores geralmente são mais alaranjadas e vermelhas.

A composição da opala tem um valor considerável de água, podendo chegar até a 21% da totalidade; na opala comum, é cerca de 6 a 10% de água. Por causa dessa tal composição aquosa, quando extraída, ela pode se quebrar se for secada muito rápido.

Os romanos acreditavam que ela era símbolo de pureza e esperança. Na mitologia, acredita-se também que quando Deus criou o mundo, ele raspou todas as cores usadas para criar a terra e criou essa pedra preciosa. Na Era Medieval, os homens acreditavam que a opala tinha poderes curandeiros; assim, usavam-na para curar doenças dos olhos.

Composição: Óxido de silíco hidratado.

Fórmula química: SiO₂nH₂O

Esmeralda e Água-marinha

A esmeralda e a água-marinha são variações do mineral berilo, e têm a mesma composição. As impurezas na composição dessas pedras que denominam suas cores, como por exemplo, o verde-aveludado das esmeraldas, que também são combinação de quantidades de crômio. O ferro também pode dar origem às cores azuis esverdeadas da água-marinha.

As esmeraldas, são as mais apreciadas e valiosas, pois as perfeitas são extremamente raras, já que a maioria das que são encontradas apresentam impurezas e "fraturas".

A água-marinha foi descoberta em Madagascar, país africano. Ela é uma gema delicada e sensível à pressão. Quando usada em joias é recomendado ter bastante cuidado, pois é uma pedra muito frágil.

Já a esmeralda, foi descoberta no deserto do Egito antigo, há cerca de 5000 anos. Ela é considerada a quinta gema mais cara do mundo, perdendo apenas para o diamante, rubi, alexandrita e safira.

Composição: Silicato de alumínio e berilo.

Fórmula química: Be₃Al₂Si₆O₁₈

Alexandrita

É uma das pedras mais raras que podem ser encontradas na natureza. Uma grande característica nessa pedra são os efeitos ópticos causados nela pela luz, que podem alternar a sua cor. De dia, a alexandrita pode ficar com tons esverdeados, amarelos, marrons, cinzas e azuis, e de noite fica mais avermelhada. Foi descoberta na Rússia, em 1883.

Muitos garimpeiros chegavam a confundir a alexandrita com a esmeralda, por isso sua descoberta é 'recente'.

Essa gema possui um grau de dureza alto, o calor excessivo também pode influenciar fortemente na mudança da sua cor.

Composição: Óxido de berílio e alumínio.

Fórmula química: BeAl₂O₄

Rubi e Safira

O rubi e a safira são pedras com cores muito intensas. As duas gemas são variações do corindo, mineral bastante duro. Assim como a maioria das pedras, suas cores derivam de impurezas em sua fórmula. O crômio deriva do vermelho intenso do rubi.

Já as pedras de outras cores derivadas do corindo são conhecidas como safira, sendo as azuis as mais famosas.

Os rubis geralmente têm formatos planos, enquanto as safiras têm formas de barris ou pirâmides.

Ambos são pleocroicos, por isso sua lapidação deve ser cuidadosa, para que apresentem uma melhor cor.

Composição: Óxido de alumínio

Fórmula química: Al₂O₃

Ametista

Também conhecida como quartzo-violeta, a ametista é uma variação da família do quartzo, geralmente de cor violeta ou púrpura, por causa da presença do ferro, do manganês e do titânio em sua composição. Se exposta ao sol, sua cor pode enfraquecer.

Ela se forma em cavidades de rochas vulcânicas ou em pegmatitos.

Seu nome vem do grego “amethystos”, que quer dizer “sóbrio”. Na antiguidade, acreditavam que essa pedra dava estabilidade, sorte e proteção.

Composição: Dióxido de silício

Fórmula química: SiO₂

Topázio

Nos tempos antigos, todas as gemas amareladas eram consideradas topázios. Foi quando, em 1737, conseguiram diferenciar os topázios das outras gemas amareladas. Suas cores podem variar entre o amarelo, alaranjado, castanho e rosa.

Seu nome vem do grego 'Topazus', que nominava uma ilha do Mar Vermelho. O significado da palavra é 'procurar', expressão que caracterizava bem a ilha, pois estava sempre rodeada de névoa e era difícil de ser encontrada.

Em sua fórmula pura é incolor, mas ganha geralmente a coloração amarela graças às impurezas. Atualmente, foram encontrados topázios de cerca de 100 quilos, na Ucrânia, país da Europa Oriental.

O topázio em forma bruta deve ter cuidados especiais, já que nessa forma, ele pode se partir em lascas facilmente.

Outras variações do topázio são o topázio imperial, que pode ser até lilás, e o topázio azul, que também pode ter outras variações de azul, como azul-celeste, azul-suíço e azul-cobalto.

Composição: Silicato de alumínio fluorado.

Fórmula química: Al₂(F,OH)₂SiO₄

Turquesa

A turquesa é uma pedra admirada desde a antiguidade, quando os egípcios começaram a minerá-la, há 6000 anos, e também foi uma das primeiras pedras a ser imitada.

Ela é um mineral de composição de fosfato mais alguns outros minerais diminutos. Ela ocorre geralmente nas rochas de regiões áridas.

Sua cor, um azul único, que até leva nome de uma variação, 'azul-turquesa', deve-se à presença de cobre em sua composição.

Na atualidade, a maioria das turquesas é produzida no sudoeste dos Estados Unidos, mas também pode ser encontrada em abundância na China e no Irã.

Composição: Fosfato hidratado de cobre e alumínio

Fórmula: CuAl₆(PO₄)₄(OH)₈·5H₂O