Turquoise is a phosphate of hydrated copper and aluminium.

Its colour 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 has been mined for almost 3,000 years. The oldest deposits are in Iran near the town of Nishapur and in Egypt in the Sinai region.

Today, Iran remains the main producer, although new mines have sprung up in Mexico and the western United States (Nevada, Arizona, Colorado, New Mexico, California, etc.).