Turquoise. Polished stones of the precious mineral turquoise (hydrated copper aluminium phosphate). These opaque gemstones are cryptocrystalline, meaning that their crystals are only visible under a microscope. Turquoise stones have been mined for use as ornamental jewellery for over 8000 years. Examples come from the early Egyptian, Persian and Native American cultures. Deposits are found in the southern USA, Australia and the Middle East.