Turquoise

Turquoise is an opaque, blue-to-green mineral that is a hydrated phosphate of copper and aluminium, with the chemical formula CuAl₆(PO
4)₄(OH)₈·4H
2O. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue. In recent times, turquoise, like most other opaque gems, has been devalued by the introduction of treatments, imitations, and synthetics onto the market.

The substance has been known by many names, but the word turquoise, which dates to the 17th century, is derived from the French turques for "Turks", because the mineral was first brought to Europe from Turkey, from mines in the historical Khorasan Province of Persia.Pliny the Elder referred to the mineral as callais and the Aztecs knew it as chalchihuitl.

Properties of turquoise

The finest of turquoise reaches a maximum hardness of just under 6, or slightly more than window glass. Characteristically a cryptocrystalline mineral, turquoise almost never forms single crystals and all of its properties are highly variable. Its crystal system is proven to be triclinic via X-ray diffraction testing. With lower hardness comes lower specific gravity (2.60–2.90) and greater porosity: These properties are dependent on grain size. The lustre of turquoise is typically waxy to subvitreous, and transparency is usually opaque, but may be semitranslucent in thin sections. Colour is as variable as the mineral's other properties, ranging from white to a powder blue to a sky blue, and from a blue-green to a yellowish green. The blue is attributed to idiochromatic copper while the green may be the result of either iron impurities (replacing aluminium) or dehydration.

Turquoise (disambiguation)

Turquoise is a gemstone.

Turquoise may also refer to:

Turquoise (trading platform)

Turquoise is an equities trading platform (Multilateral trading facility or MTF), created by nine major investment banks in 2008. The aim was to provide dealing services at a 50% discount to traditional exchanges. It is a hybrid system that allows trading both on and off traditional exchanges. The system was advertised as a "pan-European platform based in London".

History

It was set up by a consortium of banks made up of BNP Paribas, Citi, Credit Suisse, Deutsche Bank, Goldman Sachs, Merrill Lynch, Morgan Stanley, Société Générale and UBS. The group selected EuroCCP to provide clearing and settlement services. The group selected Sweden's Cinnober as its trading platform. Turquoise developed a real-time market surveillance system "to capture breaches of trading rules and root out market irregularities". The system is based on the Progress Apama complex event processing platform. Turquoise was successfully launched on 15 August 2008.

On 21 December 2009, the London Stock Exchange Group agreed to take a 60% stake in trading platform Turquoise, which had a 7% share of the market. Turquoise was merged with the LSE's trading facility Baikal Global.

Star

A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun. Other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Once the hydrogen in the core of a star is nearly exhausted, almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime and, for some stars, by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, luminosity, and spectrum respectively. The total mass of a star is the principal determinant of its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.

Stars (Simply Red album)

Stars is the fourth album by British-based pop/soul/jazz band Simply Red, released in September 1991. Five singles were released from the album, including the UK top ten hits "Stars" and "For Your Babies". The album was a worldwide success, particularly in the band's home country where it has been certified twelve times platinum and was the best-selling album of the year in the UK for both 1991 and 1992, the first album to be the best-seller in two consecutive years since Simon & Garfunkel's Bridge over Troubled Water in 1970–71. As of February 2014 it is the 14th best-selling album of all time in the UK.

Stars was also the last album to feature member Tim Kellett, who started his own band Olive after touring. It is the only Simply Red album to feature Fritz McIntyre singing lead vocals, on the tracks "Something Got Me Started" and "Wonderland".

It was on the shortlist of nominees for the 1992 Mercury Prize. In 2000 Q placed Stars at number 80 in its list of "The 100 Greatest British Albums Ever".

Stars (M. C. Escher)

Stars is a wood engraving print created by the Dutch artist M. C. Escher in 1948, depicting two chameleons in a polyhedral cage floating through space.

Although the compound of three octahedra used for the central cage in Stars had been studied before in mathematics, it was most likely invented independently for this image by Escher without reference to those studies. Escher used similar compound polyhedral forms in several other works, including Crystal (1947), Study for Stars (1948), Double Planetoid (1949), and Waterfall (1961).

The design for Stars was likely influenced by Escher's own interest in both geometry and astronomy, by a long history of using geometric forms to model the heavens, and by a drawing style used by Leonardo da Vinci. Commentators have interpreted the cage's compound shape as a reference to double and triple stars in astronomy, or to twinned crystals in crystallography. The image contrasts the celestial order of its polyhedral shapes with the more chaotic forms of biology.