Turquoise, or “Persian miracle”, is one of the most metamorphic and beautiful minerals. A distinctive feature of this stone is the unusual color with various shades of blue – from pale to heavy, sky blue, bluish, and – with greenish hue due to ferric iron impurities. The stone was first brought to Europe from Turkey, that’s why it’s named turquoise – “Turkish stone”. Besides you can still meet the turquoise under such names as callais, Aztec stone (some species of aurichalcite are called so), agapite, azure spar, Arabic stone and chalchihuitl.
Formation of turquoise
Turquoise is formed around copper deposits. Turquoise veins occur also in the phosphorus-rich shales. The mineral is usually composed of reniform, nodular or solid compounds, and rarely of tiny crystals. Besides water phosphate of copper and aluminum, turquoise may contain micro-inclusions of quartz, opal and chalcedony, sulfides and carbonates, clay minerals. Turquoise is characterized by microporosity, which significantly increases due to weathering processes. In this regard, if absorbed moisture turquoise color may become darker and denser.
Coloring of turquoise
The mineral structure can be netted or like “spider web” – stones with such texture are called “reticular turquoise”. Note that the coloring of the stones is often uneven: one can see different spots, or even black and brown streaks. Apart from the main blue turquoise color may have intermediate shades such as white, green, yellow and brown. Turquoise coloring may vary both within the same sample and around one deposit. Stones with a combined texture have different color blends and speckles that look very nice. In particular, this “motley” turquoise – interspersed with other species of reddish, brown, black, darker shades of green and blue – are very valuable for collectors.
But jewelry craftsmen value turquoise stones of bright and deep blue color the most. This type of turquoise is called «Royal Blue». Note that such stones are not widespread. In different deposits it is usually in amount of not more than 100 g per tone of mined material. Also Iranian “mature” Turquoise is good for jewelry. This stone requires practically no additional processing (dyeing, rehydration). But “young” Iranian turquoise has bluish-whitish texture, and is used to create various souvenirs, murals, mosaics, and other artistic compositions.
Natural turquoise is used by masters without advanced change of its chemical composition or texture and color, characteristics of porosity and hardness. The use of natural turquoise implies that the stone, found in nature, was only given the necessary form and the appropriate surface treatment such as polishing, grinding or turning. Note that in nature the large pieces of natural turquoise of high quality are rare. For example it may be stones not bigger than a large walnut. In general, the mineral samples have uneven color, which are then cut into fragments of pure color, or exposed to different ways of upgrading.
Color-treated turquoise is a mineral exposed to special treatment to reduce porosity, to intensify color, and protect from external factors. In ancient times the most accessible methods of color treatment of turquoise were impregnation with paraffin or wax. Nowadays for this purpose special resins and silicates are used (e.g. sodium silicate). Stabilized turquoise is a stone exposed to special treatment to reduce porosity and to prevent further discoloration. The color of such stone will be more saturated.
Reconstituted turquoise is received due to cementation with epoxy resins. This technique can improve the stone surface, especially its strength and wearing properties. And if resin with metallic fragments is used, an imitation of pyrite impregnation is received. Restored, or compressed turquoise is derived from turquoise chips, substandard debris and other waste, which are bound with epoxy resin or polystyrene. The color pigments can also be added to improve color of obtained samples.
Color-treated, stabilized, reconstructed and restored turquoise are the materials produced from natural raw materials – natural turquoise of poor quality or turquoise chips. Such a stone can be considered as an imitation of turquoise. It has brighter, pure hue and rich color along the cracks. Natural turquoise of high quality will crack when heated, and imitated one will melt into black glass or turn black. If you pass a needle along the stone surface impregnated with artificial resins, you will see scratch, but on the surface of the natural turquoise there will be no trail.
Imitation of turquoise
More extreme imitation of turquoise is a mixture of carbonates (e.g., dolomite, calcite, magnesite), which are powdered and then bonded with epoxy resin or plastic, and then dyed, since the ingoing minerals of white color. Ever since ancient times, turquoise was imitated in a primitive way: people dyed the surface of glass, porcelain, stoneware, bone or chalcedony. So-called “Viennese turquoise” was a mixture of malachite and phosphoric acid exposed to heat treatment. In 1957 turquoise Reese appeared, better known as the “neolith”. Stone had a pleasant blue color, and was the mixture of aluminum hydroxide (bayerite) and copper phosphate. Bonding agent for these components were also resins. Neo-turquoise was produced of copper phosphate and gibbsite.
The closest to the composition of a natural mineral became synthetic turquoise, which was introduced by M.Gofman in 1927. This stone is a homogeneous material, and was the closest to the best samples of Iranian turquoise.
Natural “analogues” of turquoise
In addition to the various artificial methods of producing turquoise, there are natural “analogues” of this mineral: lazulite, amazonite, chrysocolla, variscite, phosphate. These stones are similar to natural turquoise by external color.
One should not confuse natural turquoise and odontolite, better known as “bone turquoise”. Odontolite is derived from Greek word for “fossil tooth.” This mineral is composed of remnants of bone and dental tissue of mastodons and other extinct animals, dyed into a characteristic green color with glandular solutions. Due to the characteristic mesh texture and greenish hue this mineral is visually similar to turquoise, and analogously such coloring is explained with copper inclusions. But Bannister studied X-rays of blue odontolite samples from Simorr (France), which were identical to samples from Murcia (Spain). When compared to radiographs of turquoise, only external color similarity of these minerals was confirmed. Furthermore, it was found that odontolite comprises dental tissue and apatite, which is calcium phosphate. But there was neither copper, nor aluminum in the “bone turquoise”.
Deposits of turquoise
Today, this mineral is mined in Iran (Nishapur), USA (Arizona), in Mexico and the Sinai Peninsula. These deposits of turquoise are the most ancient. According to some sources, they were developed about 7-10 thousand years ago. Also, some of the largest deposits of turquoise is in Mongolia, Afghanistan, China, Israel, Kazakhstan and Uzbekistan.
The use of turquoise
The best sorts of natural turquoise are used to create jewelry. This stone looks especially good with gold and diamonds. Turquoise of lower quality is framed in silver. Turquoise with uneven coloring is inserted in various products – interior items, accessories and souvenirs, mosaic panels. In Central Asia and Iran turquoise is used with lazulite, which is characterized by more intense and bright blue hue. Note that turquoise can be used to decorate engravings, or the top layer is encrusted with gold and silver – such products are unique.