Turquoise is a phosphate of copper and aluminum hydrate.
Its color can vary from sky blue to green. The most prized turquoise is azure blue, with as few black veins as possible (due to the copper).
Turquoise can alter with time and oxidation, they change color and become darker and green. This is called dead or extinct turquoise.
Turquoise mines have been exploited for nearly 3000 years. The oldest deposits are those in Iran near the city of Nishapur and in Egypt in the Sinai region.
Today Iran remains the main producing country, however new mines have been developed. In Mexico and in the west of the United States (Nevada, Arizona, Colorado, New Mexico, California...).