Turquoise is a mineral with a hardness of 6 out of 10 on the Mohs scale of mineral hardness [?]. These Triclinicly structured gems are made of hydrated copper aluminum phosphate, their full chemical compound being Cu|Al6(PO4)4(OH)8,5H2O.
Turquoise is a hydrated phosphate of copper and aluminum in the Phosphates, Arsenates, Vanadates group, with a triclinic crystal system.
It rarely occurs as prismatic crystals. It is usually found as light-blue or green masses, nodules, and veins, sometimes filling cavities in various rocks. When it occurs as thin strips, it is tighter and brightly-colored at the center, lighter and porous on the outside.
It is light, very fragile, with good cleavage that is parallel to its prism faces, and fracture that is smoothly-curved, like that of a shell. It is translucent only in thin sections, with a luster that may be like wax, or porcelain. Its hardness corresponds to the size of turquoise grains in the crystalline aggregate, progressing from semi-hard to a consistency almost like blackboard chalk. It has a very high melting point, and is soluble in hydrochloric acid only when heated. It is also sensitive to soap, water, and grease.
When polished, untreated turquoise may be a strong blue color, but is more often pale sky blue, greenish blue or pale green.
As a mineral mass, turquoise can contain narrow veins of other material, either isolated or like a connected web; these are usually black or brown, sometimes yellowish brown. It may also contain whitish patches of foreign materials, or even tiny crystals of pyrite.
Turquoise is a secondary mineral resulting from the alteration of aluminum-bearing rocks rich in apatite and chalcopyrite, together with chalcedony and "limonite."
Exceptional sky-blue turquoise, sometimes with white markings, have been found in Iran (Kuh-i-Nishapur, Neyshabur), Egypt (Sinai peninsula) and Turkestan (Samarkand). It is also found in Australia (Queensland, Victoria, and New South Wales). Turquoise from Los Cerillos, New Mexico, and Nevada (USA) is also well-regarded, though it is less popular because it is more greenish than blue.
The best quality turquoise still comes from Iran, but no longer in large amounts. Recent deposits in the USA (Nevada, California, Arizona, New Mexico) have been increasingly exploited. Even if most of it is of low quality, it is used after being treated to improve its hardness, consistency, and color. This treatments include: Impregnation with paraffin or wax, or impregnation with colorless or colored plastic, to make the color livelier, eliminate porosity, and make material of low hardness easier to polish.
Turquoise derives its name from the French word, "pierre turquoise," meaning "Turkish stone," from the fact that it was brought to Europe from the Eastern Mediterranean by Levantine traders, generally known as Turks. It has been mined in Iran since ancient times. The Aztecs extracted it in the area that is now New Mexico. It has served as an ornament for a very long time, having been used by the Egyptians for some thousands of years B.C. But they too, imitated turquoise with ceramic material.
Turquoise is a very valuable ornamental stone. Specimens that are of lower quality are often colored artificially or "improved" with oil, wax, or plastics, which are sometimes difficult to tell from untreated high-quality stones. But when examined under a lens, the permeation of paraffin appears as small, white, opaque patches side by side with bluer, translucent areas, sometimes set against a faint background of larger indented patches.
Because of this it has become one of the most controversial gems, as much of the material being sold today has been given so many different treatments that it is completely altered from its original appearance. Imitations now outnumber genuine turquoise.
Its attractive color and physical features are usually imitated using other similarly-colored materials like non-clay ceramic material, marble, plastic, and nodules of other minerals like magnesite and howlite, that have been externally stained. Even similarly-hued natural minerals like chrysocolla and lazulite are employed as turquoise imitations.
Synthetic turquoise has also been produced, which has its own unique honeycomb structure visible under a simple lens.
But what does turquoise look like when it has not been interfered with in any way? Its surface has a uniform texture like that of unglazed china or rock that is very fine-grained.
Untreated turquoise is used almost rough, in lightly polished nodules, or more often, in the form of rounded, polished stones that are pierced for necklaces or other items of jewelry. It is also made into carved gems, cabochons, or mini-sculptures.
Like all gems which are in pastel shades, the richer-colored types are the most preferred. The favored turquoise hue is the strong sky blue, the pale greenish blue being less preferred, and the paler green even less so. Because it has a wide range of hardness, the hardest types are worth the most.
Being regularly in demand, genuine turquoise is quite highly prized in relation to other opaque gems, even if there are a lot of it on the market. Its value is exceeded only by lapis lazuli and top quality jadeite jade.