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Turquoise is an opaque blue-green mineral that has been used in jewelry since antiquity. Treated turquoise, frequently used in modern jewelry, has been altered from its original state by a process intended to improve the color and/or durability of the natural stone. There are a number of different ways that turquoise may be treated.
Treated turquoise has existed for centuries. The earliest kinds involved a fine application of oil or wax to the stone, making it shine and deepening the color. This type of treated turquoise tends to "sweat" when exposed to heat or sunlight and can become cloudy over the years; however, the treatment can sometimes be successfully reapplied.
Most American turquoise today is stabilized or bonded with a treatment of plastic, epoxy, or water glass. This type of treated turquoise retains its look and lustre better than oil or wax treated turquoise, and the stabilization process can be used on turquoise that is too unstable to benefit from the oil or wax method. Epoxy stabilization was developed in the 1950s by Colbaugh Processing of Arizona.
Native American jewelers in the Southwestern United States strengthen thin pieces of high-grade turquoise through a process termed backing. The turquoise is glued to a stronger material to prevent its cracking. Early turquoise backings were made of car battery casings and phonograph records, while modern jewelers usually use epoxy steel resin.
Some turquoise is dyed to improve its color or make it more uniform. Prussian blue is the most common dye used in this treatment. Some people do not like dyed turquoise, as they feel it is artificial, and it sometimes leaves color on the skin or fades.
A relatively new type of treated turquoise is made through the Zachary Process, which involves chemically treating the natural stone and then heating it to improve appearance and durability. Natural turquoise may also be impregnated with vaporized quartz or treated with natural chemicals and soaked in water.
Reconstituted turquoise is the lowest quality treated turquoise. Small fragments of turquoise are pulverized then bonded together, often with foreign filler material. Turquoise purists do not consider reconstituted turquoise to be a genuine gemstone.