Amethyst is a violet variety of quartz that is often used in jewellery. The name originates from the Ancient greed méthystos, translated as “intoxicated”, a reference to the belief that it could protect its owner from drunkenness. The ancient Greeks wore the amethyst and decorated drinking vessels with it, with the belef that it would prevent intoxication. It is one of several forms of quartz.
The Amethyst is a semiprecious stone, and is traditionally the birthstone for February. A purple variety of quartz, Amethyst owes its violet colour to irradiation, iron impurities and the presence of trace elements, which result in complex crystal lattice substitutions. The hardness of the mineral is the same as quartz, which is why it is suitable for jewellery use.
The Ametrine, also known as trystinem is a naturally occurring variety of quartz. It is a mixture of citrine and amethyst, with zones of purple and orange or yellow. It is mainly sourced from Bolivia, though there are deposits exploited in Brazil and India too. The colour of the zones visible are due to differing oxidation states of iron within the crystal. The different oxidation states occur because of the temperature gradient across the crystal during its formation.
Artifical ametrine can be created by differential heat treatment of the amethyst. Most emtrine in the low price segment can be assumed as stemming from synthetic material. Since 1994, a Russian laboratory has perfected the industrial production of bicoloured quartz crystals, that are later irradiated in order to bring out the typical ametrine colours. Green–yellow or golden–blue do not exist naturally.
Diamond is a metastable allotrope of carbon, where carbon atoms are arranged in a crystal structure called a diamond lattice. It is renowned for its superlative physical qualities, most of which originate from strong covalent bonds between atoms. The diamond has the highest hardness and thermal conductivity of any other bulk material, determining the major industrial application of it.
Because of its extremely rigid lattice, the diamond can be contaminated by very few types of impurities. It also has a relatively high optical dispersion, with the ability to disperse light of different colours.
Most natural diamonds are formed in the Earth´s mantle at high temperature and pressure at depths of 140 to 190 km. They are brought close to the Earth´s surface by magma through deep volcanic eruptions, which cool into igneous rocks. They can also be produced by synthetic methods, and CVD.
Special gemological techniques have been developed to distinguish natural, synthetic and diamond stimulants.
Emerald is a gemstone, a variety of the mineral beryl, coloured green by trace amounts of chromium and also sometimes vanadium. Beryl has a hardness of 7.5 – 8 pm the Mohs scale. Most emeralds have a lot of inclusions, so their toughness is generally classified as poor.
Like all other coloured gemstones, emeralds are graded using the four Cs. In the grading of coloured gemstones, colour is usually the most important criterion. However, when grading emeralds, clarity is considered a very close second. Both are necessary. To be considered a top gem, a fine emerald must not only have a pure verdant green hue, but also a high level of transparency.
In the 1960s, the American jewellery industry changed its definition of “emerald’ to also include the green vanadium–bearing beryl as emerald too. Because of this, vanadium emeralds purchased as emeralds in America, are not recognized as such in Europe and the UK. In America, the term “Columbian Emerald” distinguishes between traditional emeralds and the new vanadium kind.
Garnets are a group of silicate minerals, that have been used as gemstones and abrasives since the Bronze Age. All species of garnets have similar physical properties and crystal forms, however they differ in chemical composition. The different species are almandine, andradite, grossular, pyrope, spessartine and uvarovite. The garnets make up two different solid solution series, which are pyrope–almandine–spessartine and uvarovite–grossular–andradite.
Garnet species can be found in numerous colours such as red, orange, yellow, green, purple, brown, blue, pink, black and colourless. The rarest of these colours is the blue garnet, which was discovered in the late 1990s in Bekily, Madagascar. Garnet species´ light transmission properties can range from the gemstone–quality transparent speciments, to the different opaque varieties that are used for industrial purposes as abrasives. The mineral´s luster is categorized as vitreous (glass–like), or resinous (amber–like).
Morganite, which as often known as “pink beryl”, “rose beryl”, “pink emerald” and “cesian beryl”, is a very rare pale pink to rose coloured gem–quality variety of beryl. Orange and yellow varieties can also be found, and colour banding is common. It can be routinely treated with heat to remove yellow patches, and it is occasionally treated by irradiation in order to improve its colour.
Pink beryl of fine colour and good sizes were first discovered in 1910 on an island on the coast of Madagascar. The New York Academy of Sciences named the pink variety “morganite” after the financier J.P.Morgan.
In October 1989, one of the largest gem morganite specimems ever was found at the Bennett Quarry in Buckfield, Maine, US. Eventually called “The Rose of Maine”. The crystal which was somewhat orange in hue, was 23cm long and about 30cm across, weighing just over 23kg.
Opal is a hydrated amorphous form of silica, which is classed as a mineraloid unlike other crystalline forms of silica. This is because of its amorphous character. Opal is deposited at a relatively low temperature, and it may occur in the fissures of almost any type of rock. It is most commonly found with basalt, limonite, marl, rhyolite and sandstone.
Opal is the national gemstone of Australia. Australian opal has been often cited as accounting for over 95% of the world´s supply of precious opal, with the state of South Australia accounting for 80%.
Internally, the structure of precious opal causes light to diffract. Depending on the conditions in its formation, it can take on numerous colours. Precious opal can range from clear to white, red, orange, yellow, green, blue, magenta, pink, rose, grey, slate, brown and black. Out of these hues, the reds against black are the rarest. White and greens, on the other hand, are the most common. Optical density varies from opaque to semi–transparent.
Common opal does not show the display of colour that the precious opal exhibits.
Topaz is a silicate mineral of aluminium and fluorine. It crystallizes in the orthorhombic system, and its crystals a mainly prismatic terminated by pyramidal and other faces.
Pure topaz is colourless and transparent, but it is usually tinted by impurities. Typical topaz is usually wine red, reddish–orange, yellow, blue–brown or grey. It can also be made white, pale green, blue, gold, pink (which is rare), reddish–yellow, of opaque to translucent/transparent.
Other varieties include orange topaz, the November birthstone and state gemstone of Utah. Imperial topaz, a yellow, pink or pink–orange which can often have a bright yellow to deep golden brown hue, sometimes even violet. Blue topaz, the state gemstone of Texas, is quite rare if natural. It can be treated to produce a more desirable darker blue. Finally, mystic topaz is colourless topaz which has been artificially coated to give the desired rainbow effect.
Topaz can be found with cassiterite and fluorite in various places such as Afganistan, Australia, Brazil, Germany, Italy, Japan, Mexico, Nigeria, Norway, Pakistan, Russia, Sri Lanka, Sweden and the United States.