Turquoise (Türkis or Türkisblau German) was among the first gems to be mined, dating back as far as 6000 BC. Ancient Egyptians mined turquoise at the Wadi Maghara mines, on the Sinia Peninsula, and turquoise was also a popular gemstone with the ancient Mesopotamians, Persians, Shang Dynasty Chinese, and Native American Anasazi, and Aztec Indians. The name "turquoise" is derived from the French "pierre turquoise," or "Turkish stone," referring to the location of the caravan trade routes that originated in, or passed through Persia.
The sky-blue (aka "robin's egg blue") color of cut turquoise can change to a greenish-blue over time, as the stone oxidizes through exposure to air. The copper component that acts as the primary coloring agent in turquoise, may come from copper sulfides such as chalcopyrite or from secondary carbonates such as malachite or azurite. The phosphorus component from apatite, and the aluminum component may be derive from the presence of feldspar. Turquoise from Nevada can contain trace amounts of iron, giving it a pale green to yellow-green color.
The black, brown, or light veining in turquoise is referred to as its matrix, which can be called "cobweb," "edisonite," "egg-shell," or "spiderweb," depending on its characteristic pattern. Originally thought to be amorphous, Turquoise is cryptocrystalline, forming in the tetragonal crystal system with a pinacoid form. Typically turquoise mineralization is restricted to a relatively shallow depth of less than 60 to 70 feet, although it does occur along deeper fracture zones where secondary solutions have greater penetration.
Turquoise & Coral Bracelet by Michael Zobel (left)
Turquoise is typically found in arid regions, filling or encrusting cavities and fractures in volcanic rock, and is often associated with limonite or other iron oxides. As a secondary mineral, turquoise forms by the action of percolating acidic aqueous solutions during the weathering and oxidation of pre-existing minerals. American southwestern turquoise is associated with weathered by-products of copper sulfide deposits in or around potassium feldspar bearing porphyritic intrusives. In some occurrences alunite, potassium aluminium sulfate, is a prominent secondary mineral.
The finest of turquoise is fracturable, reaching a maximum hardness of just under 6 on the Mohs scale. Turquoise has a waxy to vitreous luster. Turquoise has a refractive index of 1.610. Due to its opacity, turquoise is cut and shaped into beads or a cabochon.
Chemical composition: CuAl6(PO4)4(OH)8·5H2O
Persian Turquoise
The ancient Persians mined turquoise as far back as 2000 BC. Persian turquoise was sometimes referred to as Meshhed Turquoise or Meshed Turquoise, referring to the mines that were located at Nishâpur (Nishapur), in the Khorassan province (in the Alimersai Mountain region of what is now the north-eastern part of Iran). Turquoise was categorized based on quality, with the finest being called Anqushtari. Lesser stones were referred to as Arabi or Barkhaneh.
American Turquoise
Turquoise has been used by the Native American Indians for over 2000 years. Pre-Columbian artifacts from the Anasazi, Aztec, and Hohokam tribes show the importance of the stone in art, religious rituals, and trade. Turquoise beaded jewelry, referred to as 'heishi,' (strung beads or shells) was popularized by the Santo Domingo and San Felipe Pueblo Indians of the American Southwest.
'Sleeping Beauty' Turquoise
The term Sleeping Beauty Turquoise (aka "Persian Turquoise") refers to a rare, matrix-free variety of turquoise that occurs in association with granite host-rock. One of the few sources for this pure-colored stone is the 'Sleeping Beauty Mountain Mine', located in Globe, Arizona. The color range for this variety is described as darker 'Royal Blue' or lighter 'Sky Blue.' The majority of 'Sleeping Beauty' material coming from the mine may have been altered or enhanced is some manner, so you should always inquire with your jeweler (see 'Enhancements' below).
Turquoise Enhancements
Turquoise is a gemstone material that is commonly enhanced to improve appearance and durability. When in doubt, you should assume that a given piece of turquoise has been treated and/or altered in some manner. The typical methods of treatment are:
- Backed Turquoise: Natural stone backed by epoxy, or plastic resin bonding agent.
- Block Turquoise: Simulant made of ceramic, plastic or other 'non-turquoise' material.
- Dyed (Chalk) Turquoise: Treated with chemical dyes to improve or even out color.
- Natural Processed Turquoise: Treated with natural chemicals then soaked in water.
- Enhanced Turquoise: Treated with chemicals, then heated to enhance durability.
- Reconstituted (Oriental) Turquoise: Pulverized, mixed with binders, pressed into solid.
- Stabilized Turquoise: Treated with epoxy or acrylic resin to bond the material.
- Waxing: Treated with paraffin wax to improve surface luster.
- Zachary Process: Treated with chemicals, then heated to enhance color and strength.
Turquoise that has been treated with the 'Zachary Process' is difficult to differentiate from untreated material. The process is designed to strengthen the material, then heat it to dissipate any traces of residual chemicals used in the treatment. Natural material can also be strengthened by impregnating it with vaporized quartz.
The up-side of 'Enhanced' or 'Natural Processed' turquoise is that the color remains stable over time as opposed to natural, untreated turquoise which can oxidize, turning green over a long period of time. Natural stone is porous, which can be detected under a loupe, as opposed to enhanced material which will be totally smooth.
Referance Credits & Suggestions for Further Study on Turquoise
1. USGS, Donald W. Olson, Turquoise from the American Southwest 
. minerals.usgs.gov
Turquoise Jewelry
