Thursday, 18 July 2013

Turquoise - Precious Stone

Turquoise - Precious StoneHydrated phosphate of copper and aluminium.

Crystal system

Triclinic

Appearance

It occurs as microcrystalline aggregates in the form of irregular, lobed or indented nodules or in thin strips (usually of no more than a few centimeters), which are more compact and strongly colored at the center lighter and porous on the outside. It is blue-white to sky blue, light greenish blue to light green. It is generally opaque, only thin pieces being translucent.

Physical properties

The hardness, which is apparently related to the grain size of the crystalline aggregate, has a surprisingly wide range, gradually descending from 5 to 6 to a consistency almost like that of blackboard chalk (this type is also whitish). The density, which also depends on the grain size and consequent porosity, varies between 2.65 and 2.90 g/cm3. Individual crystals have refractive indices of about no 1.61, ray 1.65, but the compact, micro crystalline material only gives one index, of about 1.61.

Genesis

It is a secondary or supergene mineral and is deposited by surface waters in cracks. and cavities in alumina-rich extrusive rocks, together with other similai phosphates, chalcedony and limonite. It is present also in weathering copper deposits as in Arizona.


Occurrence

It is found in Iran, where it has been minec since time immemorial, and the United States (Nevada. Arizona, California, New Mexico). Small quantities are also found in the Sinai peninsula.

The name of the gem is apparently related to the fact that it was brought to Europe from the Eastern Mediterranean by Levantine traders, generally known as Turks. It has served as an ornament for a very long time, having been used by the Egyptians some thousands of years ac. Nowadays. it is one of the most controversial gems, because much of the material sold has undergone so many different treatments that its original appearance has been completely trans-formed.

Appearance

On the rare occasions when it has not been interfered with in any way, it has a uniform surface appearance almost like that of unglazed china or very line. grained, homogeneous rock. It may be a strong blue color. but is more often pale sky blue, greenish blue or pale green. It can contain narrow veins of other material, either isolated or as a network; these are usually black or brown, though sometimes yellowish brown. It may also contain patches of whitish foreign minerals, with occasional, minute crystals of pyrite. It is used almost rough, in lightly polished nodules or, more often, in the form of spherical or summarily rounded, polished pierced stones (for necklaces and other items of jewelry). It is also made into cabochons, carved gems, or figurines. When, as very often happens, it has been impregnated with paraffin, the surface appearance under a lens is distinctive, with small, whitish, opaque patches juxtaposed with and interpenetrated by bluer, translucent areas, sometimes set against a faint pattern of larger, indented patches. Like all gems which are basically pastel in color, the richer-colored types are the most appreciated. The preferred color is strong sky blue, the pale greenish-blue being less highly prized, and the pale green even less so. Given the wide range in hardness for this gem, the hardest types with values at least in excess of 4-4.5 are obviously worth the most.

Distinctive features

Because the most striking external feature of turquoise is its color, it is readily imitated by all types of similarly colored surrogates. Ceramic material, marble, and nodules of other minerals (how-lite, magnesite) that have been externally stained, artificially colored com-pressed powders, and plastic are merely a few of the numerous substitutes currently encountered. As a rule, a few negative criteria make a rough, preliminary distinction possible:

• It is not turquoise if it appears under a lens to consist of numerous, minute grains of polygonal shape, juxtaposed in an artificial manner, with a homogeneous blue or heterogeneous light and dark blue or light blue and whitish color;

• It is not turquoise it reacts in a matter of seconds, ten at the most, to a drop of hydrochloric acid, showing fairly strong effervescence, a change in color, or obvious surface damage (all this must be verified under a lens);

• ft is not turquoise if it is warm to the touch, light like plastic, and burns with the characteristic odor of plastic when touched by a thin piece of red-hot iron wire. Apart from these distinctions, not even establishment of the basic physical properties (the hardness is extremely variable, thus not characteristic) is sufficient to identify turquoise with any certainty and this must, therefore, be left to experts.

Occurrence

Turquoises birthstonesIn ancient times, turquoise was mined in the Sinai peninsula and Iran, while certain Central American peoples, the Aztecs in particular, extracted it in what is now known as New Mexico. The best quality turquoise still comes from Iran, but in relatively small quantities. Recently, deposits in the United States (New Mexico, Arizona, California, Nevada) have been increasingly exploited. Much of the material extracted from these sources is of low quality, but it is used, nevertheless, after being treated to improve its hardness, consistency, and color.

Value

Because it has been fashionable for a number of years, turquoise is quite highly prized in relation to comparable nontransparent gems, although it is fairly plentiful on the market. Its value is exceeded only by lapis lazuli and top quality jadeite jade.

Simulants and synthetics

The ancient Egyptians apparently imitated turquoise with ceramic material. More recently, it has been so widely simulated by the most diverse materials that imitations are nowadays perhaps more plentiful on the market than turquoise itself.

Ceramic material, glass, plastic, agglomerates of artificial powders, and nodules of artificially colored rock are among the most common simulants. Attractive synthetic turquoise has also been produced for about a decade. Tins has a highly characteristic honeycomb structure, which is recognizable under a lens. What complicates matters considerably is the fact that large quantities of turquoise are put on the market which are not of gem quality, but which because of their extraordinary porosity, have been 'morelnated with various substances to improve their appearance, resistance, and ability to take polish. The most common cases are:

• Impregnation with paraffin or wax to enliven the color eliminate porosity, and make material of low hardness easier to polish. When this treatment is carried out on good quality material, which absorbs very little and is only slightly improved, it is admissible.

• Impregnation with colorless or colored plastic to enliven the color, reduce brittleness, and allow the specimen to take an exceptional polish. This treatment, which is a partial falsification, is not acceptable.

Writer – Curzio Cipriani and Alessandro Borelli


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