Opal : as a mineral

source : wikipedia.org

Opal is a mineraloid gel which is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl and basalt. The word opal comes from the Latin opalus, by Greek ὀπάλλιος opallios.

The water content is usually between three and ten percent, but can be as high as twenty percent. Opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. These color variations are a function of growth size into the red and infrared wavelengths. Opal is Australia's national gemstone.

Precious opal

Precious opal shows a variable interplay of internal colors and even though it is a mineraloid, it does have an internal structure. At micro scales precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.It is the regularity of the sizes and the packing of these spheres that determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is approximately half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. The process can be described by Bragg's Law of diffraction.

Visible light of diffracted wavelengths cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal, of which opal is the best known natural example. In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal. Potch does not show a play of color.

The veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem. An opal doublet is a thin layer of opal, backed by a swart mineral such as ironstone, basalt, or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch.

Combined with modern techniques of polishing, doublet opal produces similar effect of black or boulder opals at a mere fraction of the price. Doublet opal also has the added benefit of having genuine opal as the top visible and touchable layer, unlike triplet opals.

The triplet-cut opal backs the colored material with a dark backing, and then has a domed cap of clear quartz or plastic on top, which takes a high polish and acts as a protective layer for the relatively fragile opal. The top layer also acts as a magnifier, to emphasize the play of color of the opal beneath, which is often of lower quality. Triplet opals therefore have a more artificial appearance, and are not classed as precious opal.

Precious opal consists of spheres of silica of fairly regular size, packed into close-packed planes which are stacked together with characteristic dimensions of several hundred nm.

Common opal

Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality), resin opal which is honey-yellow with a resinous luster, wood opal which is caused by the replacement of the organic material in wood with opal, menilite which is brown or grey, hyalite is a colorless glass-clear opal sometimes called Muller's Glass, geyserite, also called siliceous sinter, deposited around hot springs or geysers and diatomite or diatomaceous earth, the accumulations of diatom shells or tests.

Other varieties of opal

Fire opals are transparent to translucent opals with warm body colors yellow, orange, orange-yellow or red and they do not usually show any play-of-color, although occasionally a stone will exhibit bright green flashes. The most famous source of fire opals is the state of Querétaro in Mexico and these opals are commonly called Mexican fire opals.

Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. It does not display pleochroism.

Sources of opal

Australia produces around 97% of the world's opal. 90% is called ‘light opal’ or white and crystal opal. White makes up 60% of the opal productions but cannot be found in all of the opal fields. Crystal opal or pure hydrated silica makes up 30% of the opal produced, 8% is black and only 2% is boulder opal.

The town of Coober Pedy in South Australia is a major source of opal. Andamooka in South Australia is also a major producer of matrix opal, crystal opal, and black opal. Another Australian town, Lightning Ridge in New South Wales, is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color). Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix. It is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit in the south.

The Virgin Valley opal fields of Humboldt County in northern Nevada produce a wide variety of precious black, crystal, white, fire, and lemon opal. The black fire opal is the official gemstone of Nevada. Most of the precious opal is partial wood replacement. Miocene age opalised teeth, bones, fish, and a snake head have been found. Some of the opal has high water content and may desiccate and crack when dried. The largest black opal in the Smithsonian Institution comes from the Royal Peacock opal mine in the Virgin Valley.

Another source of white base opal or creamy opal in the United States is Spencer, Idaho. Spencer has an open pit mine that you can visit for a fee, about 4 times a year. One business in Spencer also brings material down from the mine site to their store, so that would be opal miners can dig for their own opal, again for a nominal fee. A high percentage of the opal found there occurs in thin layers. As a result, most of the production goes into the making of doublets and triplets.

Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil (Pedro II a city in the state of Piauí), Honduras, Guatemala, Nicaragua and Ethiopia.

In late 2008, NASA announced that it had discovered opal deposits on Mars.

Historical superstitions

In the Middle Ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal. It was also said to confer the power of invisibility if wrapped in a fresh bay leaf and held in the hand.Following the publication of Sir Walter Scott's Anne of Geierstein in 1829, however, opal acquired a less auspicious reputation. In Scott's novel, the Baroness of Arnheim wears an opal talisman with supernatural powers. When a drop of holy water falls on the talisman, the opal turns into a colorless stone and the Baroness dies soon thereafter. Due to the popularity of Scott's novel, people began to associate opals with bad luck and death.Even as recently as the beginning of the 20th century, it was believed that when a Russian saw an opal among other goods offered for sale, he or she should not buy anything more since the opal was believed to embody the evil eye.

Opal is considered the birthstone for people born in October or under the sign of Libra and the star stone for people born under Scorpio.

source : http://en.wikipedia.org/wiki/Opal

Tanzanite : as a mineral

From Wikipedia, the free encyclopedia

Tanzanite is the blue/purple variety of the mineral zoisite which was discovered in the Meralani (Merelani) Hills of Northern Tanzania in 1967, near the city of Arusha. It is used as a gemstone. Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet, and burgundy depending on crystal orientation.

Tanzanite in its rough state is usually a reddish brown color. It requires artificial heat treatment to 600 °C in a gemological oven to bring out the blue violet of the stone.Tanzanite is a rare gem. It is found mostly in the foothills of Mount Kilimanjaro. The mineral is named after Tanzania, the country where it was discovered.

World's largest tanzanite

One of the most famous large tanzanite "Queen of Kilimanjaro",on 18 karat white gold

The world's largest faceted tanzanite is 737.81 carats.One of the most famous large tanzanites (242 carats) is the "Queen of Kilimanjaro". It is set in a tiara and accented with 803 brilliant cut tsavorite garnets and 913 brilliant cut diamonds. Because tanzanite is relatively soft, it is not usually set in necklaces and earrings, so the tiara is truly a rarity.The tiara is currently on display in the Gallery of Gold and Gems at the Royal Ontario Museum in Toronto, Ontario, Canada. The exhibition is from the private collection of Michael Scott, the first CEO of Apple Computers.

Factors affecting value: grading

There is no universally accepted method of grading colored gemstones. TanzaniteOne, a major commercial player in the tanzanite market, through its no-profit subsidiary, The Tanzanite Foundation, has introduced its own color grading system. The new system's color-grading scales divide tanzanite colors into a range of hues, between blue violet and violet blue.

The normal primary and secondary hues in tanzanite are blue and purple not violet. Purple is a modified spectral hue that lies halfway between red and blue. Tanzanite is a trichroic gemstone, meaning that light that enters the stone is divided into three sections each containing a portion of the visible spectrum. After heating, tanzanite becomes dichroic. The dichroic colors are red and blue. The hue range of tanzanite is blue-purple to purple-blue.

Clarity grading in colored gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible with the unaided eye (assuming 20x20 vision).The Gemological Institute of America classifies tanzanite as a Type I gemstone, meaning it is normally eye-flawless. Gems with eye-visible inclusions will be traded at deep discounts.
source : wikipedia

Topaz : as a mineral

source : Wikipedia, the free encyclopedia

Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al₂SiO₄(F,OH)₂. Topaz crystallizes in the orthorhombic system and its crystals are mostly prismatic terminated by pyramidal and other faces.

Color and varieties

Pure topaz is colorless and transparent but is usually tinted by impurities; typical topaz is wine, yellow, pale gray or reddish-orange, blue brown. It can also be made white, pale green, blue, gold, pink (rare), reddish-yellow or opaque to transparent/translucent.

Orange topaz, also known as precious topaz, is the traditional November birthstone, the symbol of friendship, and the state gemstone for the US State of Utah

Imperial topaz is yellow, pink (rare, if natural) or pink-orange. Brazilian Imperial Topaz can often have a bright yellow to deep golden brown hue, sometimes even violet. Many brown or pale topazes are treated to make them bright yellow, gold, pink or violet colored. Some imperial topaz stones can fade on exposure to sunlight for an extended period of time.Blue topaz is the Texas state gemstone. Naturally occurring Blue Topaz is quite rare. Typically, colorless, gray or pale yellow and blue material is heat treated and irradiated to produce a more desired darker blue.

Mystic topaz is colorless topaz which has been artificially coated giving it the desired rainbow effect.

Localities and occurrence

<-Topaz Mountain, Utah
Topaz is commonly associated with silicic igneous rocks of the Topaz is commonly associated with silicicgranite and rhyolite type. It typically crystallizes in granitic pegmatites or in vapor cavities in rhyolite lava flows like those at Topaz Mountain in western Utah. It can be found with fluorite and cassiterite in various areas including Ural and Ilmen mountains of Russia, in Afghanistan, Sri Lanka, Czech Republic, Germany, Norway, Pakistan, Italy, Sweden, Japan, Brazil, Mexico, Flinders Island, Australia and the United States.

Some clear topaz crystals from Brazilian pegmatites can reach boulder size and weigh hundreds of pounds. Crystals of this size may be seen in museum collections. The Topaz of Aurungzebe, observed by Jean Baptiste Tavernier measured 157.75 carats.

Colorless and light-blue varieties of topaz are found in Precambrian granite in Mason County, Texas within the Llano Uplift. There is no commercial mining of topaz in that area.

Colorless topaz, Minas Gerais, Brazil






A cut blue topaz







A red topaz





Imperial Topaz Gem







Facet Cut Topaz Gemstones in various colors, including mystic.

Etymology and historical and mythical usage

The name "topaz" is derived (via Old French: Topace and Latin: Topazus) from the Greek Τοπάζιος (Τοpáziοs) or Τοπάζιον (Τοpáziοn), the ancient name of St. John's Island in the Red Sea which was difficult to find and from which a yellow stone (now believed to be chrysolite: yellowish olivine) was mined in ancient times; topaz itself (rather than topazios) wasn't really known about before the classical era. Nicols, the author of one of the first systematic treatises on minerals and gemstones, dedicated two chapters to the topic in 1652. In the Middle Ages, the name topaz was used to refer to any yellow gemstone, but in modern times it denotes only the silicate described above.

Many modern English translations of the Bible, including the King James Version mention Topaz in Exodus 28:17 in reference to a stone in the Hoshen: "And thou shalt set in it settings of stones, even four rows of stones: the first row shall be a sardius, a topaz, and a carbuncle (Garnet): this shall be the first row."

However, because these translations as topaz all derive from the Septuagint translation topazi[os], which as mentioned above referred to a yellow stone that was not topaz, but probably chrysolite, it should be borne in mind that topaz is not meant here.The masoretic text (the Hebrew on which most modern Protestant Bible translations of the Old Testament are based) has pitdah as the gem the stone is made from; some scholars think it is related to an Assyrian word meaning flashed.More likely, pitdah is derived from Sanskrit words (पीत pit = yellow, दह् dah = burn), meaning "yellow burn". Pliny says that Topazos is a legendary island in the Red Sea and the mineral "topaz" was first mined there. The word topaz might be related to the Arabic word توباز which meant "the subject of the search" or Sanskrit word तपस् "tapas" meaning "heat" or "fire."

source : wikipeadia

World's most famous 'unseen' diamond

By Jane O'Brien
BBC News, Washington

The room was dimly lit. Armed guards stood at both entrances and enormous ironclad doors were slid shut to seal the gallery.

Nobody spoke above a whisper as we waited for the first glimpse in half a century of one of the world's most extraordinary gems.

The Wittelsbach-Graff Diamond was last seen in public at the 1958 World Exhibition in Brussels. After that, it disappeared and its whereabouts remained a mystery until Laurence Graff, a billionaire diamond dealer, bought it at auction in 2008, appending his surname.

He and his son Francois were in the gallery of the Smithsonian's National Museum of Natural History in Washington DC as the blue stone, was brought up from a secret vault and finally revealed.

Flawless beauty

"I've been privileged over the years to own some of the world's most important and famous diamonds, but I would say that the Wittelsbach-Graff is the most valuable and the most beautiful," he said.

Sitting unadorned on a silken white cloth, it glittered grey and blue in the low light. Classed as "internally flawless" it is said to have exceptional colour and becomes intense orange when viewed under ultraviolet light.

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Citrine - as a mineral

Citrine, also called citrine fortz is a variety of quartz. It ranges in color from a pale yellow to brown. Citrine has ferric impurities, and is rarely found naturally. Most commercial citrine is in fact artificially heated amethyst or smoky quartz. Brazil is the leading producer of naturally mined citrine, with much of its production coming from the state of Rio Grande do Sul.

Citrine

In ancient times, citrine was carried as a protection against snake venom and evil thoughts.

Citrine is one of three traditional birthstones for the month of November.

It is nearly impossible to tell cut citrine from yellow Topaz visibly.

Aquamarine - as a mineral

Aquamarine
Aquamarine
General
Category	beryl variety
Chemical formula	Be3Al2Si6O18[1]
Identification
Color	transparent (or can be translucent if included), greenish blue to blue green, typically light in tone[1]
Crystal system	hexagonal[1]
Cleavage	very difficult in one direction, almost never seen[1]
Fracture	conchoidal[1]
Mohs Scale hardness	7.5 - 8[1]
Luster	vitreous to resinous[1]
Polish luster	vitreous [1]
Refractive index	1.577 - 1.583 (+/- .017)[1]
Optical Properties	Double refractive, uniaxial negative[1]
Birefringence	.005 - .009[1]
Dispersion	.014[1]
Pleochroism	weak to moderate, blue and greenish blue; or different tones of blue with lighter tones associated with the optic axis direction [1]
Ultraviolet fluorescence	inert[1]
Absorption spectra	indistinct lines at 537 and 456nm. Depending on the depth of color, there is a strong line at 427nm.[1]
Specific gravity	2.72 (+.18, -.05)[1]

Aquamarine (Lat. aqua marinā, "water of the sea") is a gemstone-quality transparent variety of beryl, having a delicate blue or turquoise color, suggestive of the tint of seawater. It's closely related to the gem emerald. Colors vary and yellow beryl (heliodor), rose pink beryl (morganite), and white beryl (goshenite) are known.

Composition

Aquamarine is a beryl with a hexagonal crystal structure and a chemical formula of Be₃Al₂Si₆O₁₈, a beryllium aluminium silicate mineral. It has a specific gravity of 2.68 to 2.74 and a Mohs hardness of from 7.5 to 8. Aquamarine typically is on the low end of the specific gravity range, normally at less than 2.7. The pink variety exhibits a high specific gravity of around 2.8. Refractive indices range around 1.57 to 1.58.

Locations of deposits

It occurs at most localities which yield ordinary beryl, some of the finest coming from Russia. The gem-gravel placer deposits of Sri Lanka contain aquamarine. Clear yellow beryl, such as occurs in Brazil, is sometimes called aquamarine chrysolite. When corundum presents the bluish tint of typical aquamarine, it is often termed Oriental aquamarine.

In the United States, aquamarines can be found at the summit of Mt. Antero in the Sawatch Range in central Colorado. In Brazil, there are mines in the states of Minas Gerais, Espírito Santo and Bahia. Zambia also produces nice aquamarine as well as Madagascar, Malawi, Tanzania and Kenya. Much of today's aquamarine is heated to give it a better color blue. The deeper the blue color, the more valuable the gem is considered.

The biggest aquamarine ever mined was found at the city of Marambaia, Minas Gerais, Brazil, in 1910. It weighed over 110 kg, and its dimensions were 48.5 cm long and 42 cm in diameter.

Aquamarine is the official state gem of Colorado.

Culture and historical/mythical usage

• Aquamarine (along with Bloodstone) is the birthstone associated with March. It is also the gemstone for the 19th Anniversary.
• People in the Middle Ages thought that aquamarine could magically overcome the effects of poison.
• Ancient sailors traveled with aquamarine crystals, believing that it would ensure a safe passage, and guarantee a safe return; they often slept with the stones under their pillow to ensure sound sleep. They believed the siren’s (mermaid) fish-like lower body was made of aquamarine.

source : http://en.wikipedia.org/wiki/Aquamarine

Amethyst : as a mineral

Amethyst is a purple variety of quartz often used as an ornament. The name comes from the Greek a ("not") and methustos ("to intoxicate"), a reference to the belief that the stone protected its owner from drunkenness; the ancient Greeks and Romans wore amethyst and made drinking vessels of it in the belief that it would prevent intoxication.

Quartz
General
Category	Mineral variety
Chemical formula	Silica (silicon dioxide, SiO2)
Identification
Color	purple
Crystal habit	6-sided prism ending in 6-sided pyramid (typical)
Crystal system	rhombohedral class 32
Twinning	Dauphine law, Brazil law and Japan law
Cleavage	None
Fracture	Conchoidal
Mohs Scale hardness	7 - lower in impure varieties
Luster	Vitreous/glossy
Refractive index	nω = 1.543 - 1.553 nε = 1.552 - 1.554
Optical Properties	Uniaxial (+) (Positive)
Birefringence	+0.009 (B-G interval)
Pleochroism	None
Streak	White
Specific gravity	2.65 constant; variable in impure varieties
Melting point	1650 (±75) °C
Solubility	H2O insoluble
Diaphaneity	Transparent to translucent
Other Characteristics	Piezoelectric

Chemistry

Amethyst is the purple variety of quartz, its chemical formula is SiO₂.

In the 20th century, the color of amethyst was attributed to the presence of manganese. However, since it is capable of being greatly altered and even discharged by heat, the color was believed by some authorities to be from an organic source. Ferric thiocyanate was suggested, and sulfur was said to have been detected in the mineral.

More recent work has shown that amethyst's coloration is due to ferric iron impurities.^[1] Further study has shown a complex interplay of iron and aluminium is responsible for the color.^[2]

On exposure to heat, amethyst generally becomes yellow, and much of the citrine, cairngorm, or yellow quartz of jewelry is said to be merely "burnt amethyst." Veins of amethystine quartz are apt to lose their color on the exposed outcrop^{[citation needed]}.

Synthetic amethyst is made to imitate the best quality amethyst. Its chemical and physical properties are so similar to that of natural amethyst that it cannot be differentiated with absolute certainty without advanced gemological testing (which is often cost prohibitive). There is one test (which is not 100 percent certain) based on "Brazil law twinning" (a form of quartz twinning where right and left hand quartz structures are combined in a single crystal^[3] which can be used to identify synthetic amethyst rather easily. In theory however it is possible to create this material synthetically as well, but this type is not available in large quantities in the market.^[4]

Composition

Amethyst is composed of an irregular superposition of alternate lamellae of right-handed and left-handed quartz. It has been shown that this structure may be due to mechanical stresses. As a consequence of this composite formation, amethyst is apt to break with a rippled fracture or to show "thumb markings," and the intersection of two sets of curved ripples may produce on the fractured surface a pattern something like that of "engine turning." Some mineralogists, following Sir David Brewster, apply the name of amethyst to all quartz which exhibits this structure, regardless of color.

Because it has a hardness of 7 on the Mohs scale, amethyst is treasured for its use in jewelry.

Hue and Tone

Amethyst occurs in a continuum of primary hues from a light slightly pinkish violet to a deep grape purple. Amethyst may exhibit one or both secondary hues, red and/or blue. The ideal grade is called "Deep Siberian" and has a primary purple hue of around 75 - 80 percent, 15-20 percent blue and (depending on the light source) red secondary hues.^[4]

A polished amethyst gem stone.

History

Amethyst was used as a gemstone by the ancient Egyptians and was largely employed in antiquity for intaglios. Beads of amethyst are found in Anglo-Saxon graves in England. It is a widely distributed mineral, but fine, clear specimens that are suitable for cutting as ornamental stones are confined to comparatively few localities. Such crystals occur either in the cavities of mineral-veins and in granitic rocks, or as a lining in agate geodes. A huge geode, or "amethyst-grotto," from near Santa Cruz in southern Brazil was exhibited at the Düsseldorf, Germany Exhibition of 1902. Many of the hollow agates of Brazil and Uruguay contain a crop of amethyst crystals in the interior. Much fine amethyst comes from Russia, especially from near Mursinka in the Ekaterinburg district, where it occurs in drusy cavities in granitic rocks. Many localities in India yield amethyst.

Museum-quality piece of Amethyst

Geographic distribution

Amethyst occurs at many localities in the United States, but these specimens are rarely fine enough for use in jewelry. Among these may be mentioned Amethyst Mountain, Texas; Yellowstone National Park; Delaware County, Pennsylvania; Haywood County, North Carolina; Deer Hill and Stow, Maine. It is found also in the Lake Superior region. Amethyst is relatively common in northwestern Ontario, and in various locations throughout Nova Scotia, but uncommon elsewhere in Canada. Amethyst is produced in abundance from the state of Minas Gerais in Brazil where it occurs in large geodes within volcanic rocks. It is also found and mined in South Korea. The largest opencast amethyst vein in the world is in Maissau, Lower Austria.

6 carat (1.2 g) pear shape amethyst ring

Value

Traditionally included in the cardinal, or most valuable, gemstones (along with diamond, sapphire, ruby, and emerald), amethyst has lost much of its value due to the discovery of extensive deposits in locations such as Brazil. The highest grade Amethyst (called "Deep Russian") is exceptionally rare and therefore its value is dependent on the demand of collectors when one is found. It is however still orders of magnitude lower than the highest grade sapphires or rubies (Padparadscha Sapphire or "Pigeon Blood" Ruby) which can go for as much as $50,000 or more per carat.^[4]

Amethyst such as the "Russian" quality (bluish purple key color) and such as material recently found in Georgia since 2005 (with some of the stones showing both blue hue and red flashes) and of which its top material is considered of the best quality presently available ^[5] still command prices of not more than $50 per carat.^[6]

Amethyst in folklore and astrology

Amethyst is the birthstone associated with February. It is also associated with the astrological signs of Pisces, Aries (especially the violet and purple variety), Aquarius, and Sagittarius. It is a symbol of heavenly understanding, and of the pioneer in thought and action on the philosophical, religious, spiritual, and material planes. Ranking members of the Roman Catholic Church traditionally wear rings set with a large amethyst as part of their office.

The Greek word "amethystos" (αμέθυστος) basically can be translated as "not drunken." Amethyst was considered to be a strong antidote against drunkenness, which is why wine goblets were often carved from it. Supposedly, when a drunken Dionysus was pursuing a maiden called Amethystos, who refused his affections, she prayed to the gods to remain chaste. The goddess Artemis granted the prayer, transforming her into a white stone; humbled by Amethystos' desire to remain chaste, Dionysus poured wine over the stone she had become as an offering, dyeing the crystals purple.

Variants of the story include that Dionysus, the god of intoxication, had been insulted by a mortal and swore revenge on the next mortal who crossed his path, creating fierce tigers to carry out his wish; the mortal turned out to be a beautiful young woman, Amethystos, who was on her way to pay tribute to Artemis. Her life is spared by Artemis, who transforms the maiden into a statue of pure crystalline quartz to protect her from the brutal claws. Dionysus wept tears of wine in remorse for his action at the sight of the beautiful statue. The god's tears stained the quartz purple.^[7] Another variation involves the goddess Rhea presenting Dionysus with the amethyst stone to preserve the winedrinker's sanity.^[8]

Notes

1. ^ Klein, Cornelis and Hurlbut, Cornelius S., 1985 Manual of Mineralogy (after JD Dana) 20th edition, p. 441, John Wiley & Sons, New York
2. ^ Cohen, Alvin J., 1985, Amethyst color in quartz,the result of radiation protection involving iron', American Mineralogist, V. 70, pp 1180-1185
3. ^ Quartz Page Twinning Crystals. Retrieved on 2007-05-28.
4. ^ ^{a b c} Secrets of the Gem Trade; The Connoisseur's Guide to Precious Gemstones Richard W Wise, Brunswick House Press, Lenox, Massachutes., 2003
5. ^ Bowling, Ryan L (2005). "Jackson Crossroads Amethyst Deposit, Wilkes County, Georgia". Mineralogical Record A 36 (6): 479(8). Retrieved on 2007-05-27.
6. ^ Mountain Gems and Minerals, Little Switzerland, NC; one of the authors or Mineralogical Record and other articles and co-owner of the mine. Tucson showreport images: http://www.gamineral.org/t05-clarion_hotel.html
7. ^ source
8. ^ (Nonnus, Dionysiaca, XII.380)

source : http://en.wikipedia.org/wiki/Amethyst

Ruby : as a mineral

Ruby is a light pink to blood red gemstone, a variety of the mineral corundum (aluminium oxide). The common red color is caused mainly by the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. It is considered one of the four precious stones, together with the sapphire, the emerald and the diamond. Improvements used include color alteration, improving transparency by dissolving rutile inclusions, healing of fractures (cracks) or even completely filling them.

Prices of rubies are primarily determined by color (the brightest and best "red" called Pigeon Blood Red, command a huge premium over other rubies of similar quality). After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions will indicate the stone has been treated one way or another. Cut and carat (size) also determine the price.

more about ruby : http://en.wikipedia.org/wiki/Ruby

Ruby
Ruby crystal before faceting, length 0.8 inches (2 cm)
General
Category	Mineral variety
Chemical formula	aluminium oxide with chromium, Al2O3::Cr
Identification
Color	Red, may be brownish or purplish
Crystal habit	Varies with locality. Terminated tabular hexagonal prisms.
Crystal system	Trigonal
Cleavage	No true cleavage
Fracture	Uneven or conchoidal
Mohs Scale hardness	9.0
Luster	Vitreous
Refractive index	~1.762-1.770
Pleochroism	Orangey Red, Purplish Red
Ultraviolet fluorescence	red under longwave
Streak	white
Specific gravity	4.0
Melting point	2050°C
Fusibility	perfectly
Solubility	none
Diaphaneity	transparent

Emerald : as a mineral

Emerald
Emerald with host rock
General
Category	Silicate mineral
Chemical formula	Beryllium aluminium silicate with chromium, Be3Al2(SiO3)6::Cr
Identification
Color	Green
Crystal habit	Hexagonal Crystals
Crystal system	Hexagonal
Cleavage	Poor Basal Cleavage (Seldom Visible)
Fracture	Conchoidal
Mohs Scale hardness	7.5 - 8.0
Luster	Vitreous
Refractive index	1.576 - 1.582
Pleochroism	Distinct, Blue-Green/Yellow-Green
Streak	White
Specific gravity	2.70 - 2.78

Emeralds are a variety of the mineral beryl, colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5 - 8 on the 10 point Mohs scale of mineral hardness. Most emeralds are highly included, so their brittleness (resistance to breakage) is classified as generally poor. The origin of the word "emerald" is said to be a Sanskrit word meaning "green".

Properties determining value

Emeralds come in many shades of green and bluish green. There is a wide spectrum of clarity, dependent on the inclusions and fractures in the crystal. Clear stones with dark yet vibrant color command the highest prices. Almost all emeralds contain numerous flaws, cracks, and inclusions, which can negatively affect the clarity. These are given the name "jardin", from the French word for garden. The value of an emerald depends on cut, color, clarity, and carat. Currently the best emeralds come from the Muzo mine in Colombia.

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. The U.S. Federal Trade Commission requires the disclosure of this treatment when a treated emerald is sold.The amount of oil entering an emerald microfissure is roughly equivalent to the size of a period (full stop) in print^{[citation needed]}. The use of green-tinted oil is generally not considered acceptable by the gem trade.

Emerald localities

Emeralds in antiquity were mined by the Egyptians and in Austria, as well as Swat in northern Pakistan.

A rare type of emerald known as a trapiche emerald is occasionally found in the mines of Colombia. A trapiche emerald exhibits a "star" pattern; it has raylike spokes of dark carbon impurities that give the emerald a six-pointed radial pattern. It is named for the trapiche, a grinding wheel used to process sugarcane in the region. Colombian emeralds are generally the most prized due to their transparency and fire. Some of the most rare emeralds come from three main emerald mining areas in Colombia: Muzo, Coscuez, and Chivor. Fine emeralds are also found in other countries, such as Zambia, Brazil, Zimbabwe, Madagascar, Pakistan, India, Afghanistan and Russia. In the US, emeralds can be found in North Carolina. In 1998, emeralds were discovered in the Yukon, Canada .

Synthetic emerald

Emerald showing its hexagonal structure

Emerald is a rare and valuable gemstone and, as such, it has provided the incentive for developing synthetic emeralds. 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. Because Chatham's emeralds do not have any water and contain traces of vanadate, molybdenum and vanadium, a lithium vanadate flux process is probably involved. The other large producer of flux emeralds is Pierre Gilson Sr., which has been on the market since 1964. Gilson's emeralds are usually grown on natural colorless beryl seeds which become coated on both sides. Growth occurs at the rate of 1 mm per month and a typical seven-month growth run produces emerald crystals of 7 mm of thickness (Nassau, K. Gems Made By Man, 1980).

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 Inbruck, 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 on acid conditions. Later, from 1965 to 1970, the Linde Division of Union Carbide produced completely synthetic emeralds by hydrothermal synthesis. According to their patents (US3,567,642 and US3,567,643), 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 in order to prevent nucleation and confine growth to the seed crystals. Growth occurs by a diffusion-reaction process, assisted by convection. Typical growth conditions include pressures of 700-1400 bars at temperatures of 500 to 600 °C with a temperature gradient of 10 to 25 °C. Growth rates as fast as 1/3 mm per day can be attained^{[citation needed]}

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.

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."

Wispy veil-like inclusions are common in flux-grown synthetic emeralds.

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 DC.

Emerald is regarded as the traditional birthstone for May, as well as the traditional gemstone for the astrological signs of Taurus and Cancer. One of the more quaint anecdotes on emeralds was by the 16th century historian Brantome, who referred to the many impressive emeralds the Spanish under Cortez had brought back to Europe from Latin America. On one of Cortez most famous emeralds he had the text engraved Inter Natos Mulierum non sur-rexit mayor (Among them borne of woman there hath not arisen a greater Man. XI, 11) which referred to John the Baptist. Brantome considered engraving such a beautiful and simple product of nature sacrilegious and considered this act the cause for Cortez 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 who soon died after.

High Priest Breastplate

In Exodus chapters 28 and 39, a number of precious stones are mentioned to be placed in the High Priest's Breastplate, representing the different tribes of Israel. This is generally considered to be (one of) the origin(s) for our present day tradition of birthstones.

According to Rebbenu Bachya, and the King James Version, the Hebrew word Nofech in Exodus 28:18 means Emerald, and was the stone on the Hoshen representing the tribe of Judah. However, the Septuagint translates the word as Anthrax, meaning coal, probably in reference to the colour of burning coal, and therefore many rabbinical sources, and most scholars, consider Nofech to mean a red garnet – traditionally called a carbuncle, which happens to be the Vulgate's translation of the word. There is a wide range of views among traditional sources about which tribe the stone refers to.

There are many complexities to identifying the Emerald as being the third stone or perhaps another stone on the breast plate. Multiple translations of the bible have created confusion about the nomenclature of the different stones. Another important fact is that in actuality there are 2 different breastplates made within a period of 800 years, and where it is assumed the first breastplate did not carry an Emerald but a green Felspar, and a real Emerald in the second breastplate. Finally the 12th stone in the Breastplate (which in the original text was actually listed as the 6th stone) has more generally been identified as Beryl which was already included in the group of stones generally referred to as Smaragdus by Theophrastus in the Greek era. . Further unreferenced claims regarding the possibility of what gemstone the Emerald could really have been include Jasper, and even Rubies.

In some cultures, the emerald is the traditional gift for the 55th wedding anniversary. It is also used as a 20th and 35th wedding anniversary stone.

Famous emeralds

• Gachala Emerald (origin: Colombia)
• Chalk Emerald (origin: Colombia)
• Nidvin Emerald (origin: Colombia)
• Duke of Devonshire Emerald (origin: Colombia)
• Mackay Emerald
• Greenshorkire Emerald
• Edward the Confessor Emerald in the Imperial State Crown of Great Britain

source : wikipedia http://en.wikipedia.org/wiki/Emerald

Sapphire : as a mineral

General
Category	Mineral Variety
Chemical formula	aluminium oxide, Al2O3
Identification
Color	Every color except red (which is ruby)
Crystal habit	massive and granular
Crystal system	Trigonal
Cleavage	None
Fracture	Conchoidal, splintery
Mohs Scale hardness	9.0
Luster	Vitreous
Refractive index	1.762-1.778
Pleochroism	Strong
Streak	White
Specific gravity	3.95-4.03
Fusibility	infusible
Solubility	insoluble
Other Characteristics	Coefficient of thermal expansion 5e-6–6.6e-6/K

Sapphire refers to gem varieties of the mineral corundum, an aluminium oxide (Al2O3), when it is a color other than red. It can be found naturally or manufactured in large crystal boules for varied applications, including infrared optical components, watch faces, high-durability windows, and wafers for the deposition of semiconductors such as GaN nanorods.

The mineral corundum consists of pure aluminium oxide. Trace amounts of other elements such as iron, titanium and chromium give corundum their blue, yellow, pink, purple, orange or greenish color. Sapphire includes any gemstone quality varieties of the mineral corundum except the fully saturated red variety, which is instead known as ruby, and the pinkish-orange variety known as padparadscha.

source :wikipedia,the free encyclopedy
more information at : http://en.wikipedia.org/wiki/Sapphire

Diamond : as a mineral

Diamond
A scattering of round-brilliant cut diamonds shows off the many reflecting facets.
General
Category	Native Minerals
Chemical formula	C
Identification
Molecular Weight	12.01 u
Color	Typically yellow, brown or gray to colorless. Less often in blue, green, black, translucent white, pink, violet, orange, purple and red.
Crystal habit	Octahedral
Crystal system	Isometric-Hexoctahedral (Cubic)
Cleavage	111 (perfect in four directions)
Fracture	Conchoidal - step like
Mohs Scale hardness	10[1]
Luster	Adamantine
Polish luster	Adamantine
Refractive index	2.4175–2.4178
Optical Properties	Singly Refractive
Birefringence	none
Dispersion	.044
Pleochroism	none
Ultraviolet fluorescence	colorless to yellowish stones - inert to strong in long wave, and typically blue. Weaker in short wave.
Absorption spectra	In pale yellow stones a 415.5 nm line is typical. Irradiated and annealed diamonds often show a line around 594 nm when cooled to low temperatures.
Streak	White
Specific gravity	3.52 (+/- .01)
Density	3.5-3.53
Diaphaneity	Transparent to subtransparent to translucent

Diamond is an allotrope of carbon. It is the hardest known natural material and the third-hardest known material after aggregated diamond nanorods and ultrahard fullerite. Its hardness and high dispersion of light make it useful for industrial applications and jewelry.

Diamonds are specifically renowned as a material with superlative physical qualities; they make excellent abrasives because they can be scratched only by other diamonds, Borazon, ultrahard fullerite, or aggregated diamond nanorods, which also means they hold a polish extremely well and retain their lustre. Approximately 130 million carats (26,000 kg) are mined annually, with a total value of nearly USD $9 billion, and about 100,000 kg are synthesized annually.^[2]

The name diamond derives from the ancient Greek adamas (αδάμας; “invincible”). They have been treasured as gemstones since their use as religious icons in ancient India and usage in engraving tools also dates to early human history.^[3][4] Popularity of diamonds has risen since the 19th century because of increased supply, improved cutting and polishing techniques, growth in the world economy, and innovative and successful advertising campaigns. They are commonly judged by the “four Cs”: carat, clarity, color, and cut.

Roughly 49% of diamonds originate from central and southern Africa, although significant sources of the mineral have been discovered in Canada, India, Russia, Brazil, and Australia. They are mined from kimberlite and lamproite volcanic pipes, which brought to the surface the diamond crystals from deep in the Earth where the high pressure and temperature enables the formation of the crystals. The mining and distribution of natural diamonds are subjects of frequent controversy such as with concerns over the sale of conflict diamonds (aka blood diamonds) by African paramilitary groups.

references :
1.^ ^{a b c d e f g h i j k} Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
2.Yarnell, Amanda (2004). "The Many Facets of Man-Made Diamonds". Chemical and Engineering News 82 (5): 26–31. American Chemical Society. ISSN 0009-2347. Retrieved on 2006-10-03
3.Pliny the Elder. Natural History: A Selection. Penguin Classics, p. 371. ISBN 0140444130
4."Chinese made first use of diamond", BBC News, 17 May 2005. Retrieved on 2007-03-21
5.Wikipedia,the free encyclodepia