Chemically, a hydrated phosphate of copper and aluminum. Turquoise is usually found in the"alteration zones," of arid or desert regions. These zones are areas where the native, original rocks have been altered through the intrusion of other rocks from some volcanic or other thermal influence. The hydrothermal alteration is created by magma solutions from deep in the earth being forced to the surface through fractures or pores which eventually change the original rocks.
Several steps and processes are necessary to create turquoise. First there must be a source of copper. This occurs in a rather limited number of areas in the world. There must be a source of phosphorus CO-located with the copper. Usually from the mineral, Apatite. Which is not always in rocks associated with copper.
There must also be feldspar for the aluminum. Along with deep hydrothermal alteration. Which breaks down the feldspars and frees the aluminum needed for the turquoise. The phosphorus usually comes from phosphoric acid leached from the Apatite, during the hydrothermal alteration.

The copper is usually introduced into the "host" rocks by the rising hot magma. The copper readily oxidizes near the surface when it is in the hot magma solution. It reacts freely with the aluminum and phosphoric acid to form turquoise.
At this time other minerals enter into the turquoise structure and create color variations. The chemical formula of turquoise is: (CuAl₆ (PO4)₈ 4H2O) this structure will very greatly with the introduction of Iron, Calcium, Magnesium, Manganese, Silicon, and Zinc. These additional elements when incorporated in the molecular structure of turquoise influence its color and hardness. The color of turquoise can vary from a deep blue to a deep green, with every variation of color in-between.
Generally, the more copper in the molecular structure the bluer the turquoise. The introduction of iron causes the greener cast to the stone. Turquoise creation is affected by many other factors too. For example; the best, hardest turquoise is found within 100 feet of the earth's surface.
This is not to say that turquoise has not been found in areas without igneous or volcanic activity. Turquoise has been found in the Sinai and in Australia. In these two areas it is found in sandstone and shale.

Its One other key geological activity is called silicification. It too is an act of hydrothermal and intrusive alteration. Here silica, which is a common associate of turquoise, is introduced into the turquoise deposit. This addition and periods of intense heat are responsible for the hardness of the turquoise and frequently the matrix as well.Turquoise is a relatively soft gemstone, and can be easily scratched and broken. This porous opaque stone is easily discolored by oil and pigments, and changes color when it loses some of its water content. A sky blue shade in turquoise is due to the presence of copper, while iron gives it a greener tone. Ochre and brown-black veins in the stone occur during the formation of turquoise, caused by inclusions from nearby rock fragments or from oxide staining. The most valued variety of turquoise is an intense sky blue color, like the color of a robin's egg. Hard, relatively non-porous compact stones have the best appearance because the stone can be finely polished. Pale and chalky varieties however are sometimes impregnated with oil, paraffin, liquid plastic and glycerin to give it a good polish.

Turquoise - Main Characteristics