TURQUOISE

Turquoise is an opaque, blue-to-green mineral. It is rare and valuable in finer grades and has been prized as a gem and ornamental 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, some difficult to detect even by experts.

The substance has been known by many names, but the word turquoise was derived around 16th century from the French language either from the word for Turkish (Turquois) or dark-blue stone (pierre turquin). This may have arisen from a misconception: turquoise does not occur in but was traded at Turkish bazaars to Venetian merchants who brought it to Europe. The colour, however, has been employed extensively in the decorative tiles adorning Turkish places of worship and homes for hundreds of years, beginning with the Seljuks, and the association quite possibly has caused the name to take root.

Even the finest of turquoise is fracturable. 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.

Treatments

Turquoise is treated to enhance both its colour and durability (i.e., increased hardness and decreased porosity). Historically, light waxing and oiling were the first treatments to be used (since ancient times), providing a wetting effect (thereby enhancing the colour and lustre); this treatment is more or less acceptable by tradition, and because such material is usually of a higher grade to begin with. Conversely, the later development of pressure impregnation of otherwise unsaleable chalky American material by epoxy and plastics (such as polystyrene) and water glass—also producing a wetting effect in addition to improving durability—are rejected by some as too radical an alteration.

Plastic and water glass are technologically superior to oil and wax in that the former treatment are far more permanent and stable, and can be applied to material too friable for oil or wax to be of sufficient help; such material is termed "bonded" or "stabilized" turquoise. The epoxy binding technique was first developed in the 1950s and has been attributed to Colbaugh Processing of Arizona, a company that still operates today. The majority of American material is now treated in this manner; although it is a costly process requiring many months to complete, without impregnation most American mining operations would be unprofitable.

Oiled and waxed stones are also prone to "sweating" under even gentle heat or if exposed to too much sun, and they may develop a white surface film or bloom over time. (With some skill, oil and wax treatments can be restored.) Likewise, the use of Prussian blue and other dyes—often in conjunction with bonding treatments—to enhance (that is, make uniform or completely change) colour is regarded as fraudulent by purists—especially since some dyes may fade or rub off on the wearer. Dyes have also been used to darken the veins of turquoise. Perhaps the most radical of treatments is "reconstitution", wherein supposedly fragments of fine material too small to be used singly are powdered and then bonded to form a solid mass. Much (if not all) of this "reconstituted" material is likely a complete fabrication (with no natural components), or may have foreign filler material added to it (see Imitations section). Another treatment—the details of which remain undisclosed—is the so-called Zachery process, named after its developer, electrical engineer and turquoise trader James E. Zachery. This process claims to use only medium grade material at a minimum, leaving the turquoise harder and with a better colour and lustre.

As the finer turquoise is often found as thin seams, it may be glued to a base of stronger foreign material as a means of reinforcement. These are termed "Backed" and all turquoise cut in the southwest is backed and is the standard.Indians because of their considerable use and wearing of turquoise found that backing increased the wearability of the turquoise. They found if the stone was not backed it would for the most part end up cracking. Early backing materials were the cassing of old model T batteries and progressed to old phonograph records and then to the final use of epoxy steel resins.This is a very helpful way of determening the age of old Indian jewelry.While backing is not known outside of the Indian and southwest jewelry trade. It is the standard inside of it.All turquoise cut for this trade is backed and to see turquoise that is not backed is considered cut by inexsperience or cut oversea's.Backed turquoise of highest quality is not discounted because it is backed.Backing does not decrease value.

As is so often with gems, full disclosure is frequently not given; it is therefore left to gemmologists to detect these treatments in suspect stones, using a variety of testing methods—some of which are necessarily destructive. For example, the use of a heated probe applied to an inconspicuous spot will reveal oil, wax, or plastic treatment with certainty.

Valuation and care

Richness of colour is the chief determiner of value in turquoise; generally speaking, the most desirable is a strong sky to "robin's egg" blue. Value decreases with the increase of green hue, lightening of colour, and mottling. In Tibet, however, a greener blue is said to be preferred. Whatever the colour, turquoise should not be excessively soft or chalky; even if treated, such lesser material (to which most turquoise belongs) is liable to fade or discolour over time and will not hold up to normal use in jewellery.

The mother rock or matrix in which turquoise is found can often be seen as splotches or a network of brown or black veins running through the stone in a netted pattern; this veining may add value to the stone if the result is complimentary, but such a result is uncommon. Such material is sometimes described as "spiderweb matrix"; it is most valued in the Southwest United States and Far East, but is not highly appreciated in the Near East where unblemished and vein-free material is ideal (regardless of how complimentary the veining may be). Uniformity of colour is desired, and in finished pieces the quality of workmanship is also a factor; this includes the quality of the polish and the symmetry of the stone. Calibrated stones—that is, stones adhering to standard jewellery setting measurements—may also be more sought after. Like coral and other opaque gems, turquoise is commonly sold at a price according to its physical size in millimetres rather than weight.

Turquoise is treated in many different ways, some more permanent and radical than others. Controversy exists as to whether some of these treatments should be acceptable, but one can be more or less forgiven universally: This is the light waxing or oiling applied to most gem turquoise to improve its colour and lustre; if the material is of high quality to begin with, very little of the wax or oil is absorbed and the turquoise therefore does not "rely" on this impermanent treatment for its beauty. All other factors being equal, untreated turquoise will always command a higher price. Bonded and "reconstituted" material is worth considerably less.

Being a phosphate mineral, turquoise is inherently fragile and sensitive to solvents; perfume and other cosmetics will attack the finish and may alter the colour of turquoise gems, as will skin oils, as will most commercial jewelry cleaning fluids. Prolonged exposure to direct sunlight may also discolour or dehydrate turquoise. Care should therefore be taken when wearing such jewels: cosmetics, including sunscreen and hairspray, should be applied before putting on turquoise jewellery, and they should not be worn to a beach or other sun-bathed environment. After use, turquoise should be gently cleaned with a soft cloth to avoid a build up of residue, and should be stored in its own box to avoid scratching by harder gems. Also, make sure the box is not airtight, or the turquoise will become ruined