When exposed to the elements in the open air, turquoise weathers, decomposing into a dusty white powder. It is in the nature of turquoise to change. But the color is inconstant and unstable and can fade to green. While turquoise can have many shades, including a common pale green caused by traces of iron, the finest stones, deemed gems-those aged for thousands of years in the oldest rocks-are sky blue. 5 Contact with copper ores and the passing of time give turquoise, an opaque stone with a soft surface and a high index of light refraction, its defining physical property-a sky-blue color.
4 All of this slow, nearly motionless change occurs in the zone of oxidation, in the first few hundred feet below the crust of the earth.Īs these minerals solidify into rocks, they enclose the now crystallized turquoise, a naturally occurring mineral substance with a definite chemical composition. 3 Waters containing phosphoric acid that flow through aluminous rocks decompose feldspar crystals into kaolin, a mineral substance that is associated with turquoise, freeing necessary aluminum silicates. As rainwater seeps from the surface through these rocks, it breaks down and converts their minerals into new chemical substances. Over centuries, deep-seated weathering and exposure to the elements alter their form. Igneous rocks rich in aluminum and copper minerals give birth to turquoise. If turquoise is mined before it has aged, its color fades. " 2 To reach the state at which it can be cut from its rock matrix and traded as a precious stone and ornament, it must undergo deep geological alterations in the layers of the earth.
But all the cherry needs to ripen is the sun of one season, whereas for turquoise it takes one thousand years. 1 An opaque stone whose pale blue results from the presence of copper, turquoise has surfaced in desert environments from Eurasia to Mesoamerica, been valued as a celestial object, and been traded across the world.Ī nineteenth-century Persian natural history compares the formation of turquoise to the ripening of fruit: "It is said turquoise is like a cherry-the more it ripens, the better. In a geological process that lasts thousands of years, nature weathers, buries, and erodes these rocks, bringing their copper, aluminum, phosphorus, oxygen, hydrogen, and water together to create the chemistry of turquoise, CuAl 6(PO 4) 4(OH) 8♴H 2O (see table 1). A hydrous phosphate of copper and aluminum, it is born in igneous rocks, as magma, fiery liquid deep within the earth, surges toward the surface, pools, and solidifies. Turquoise is a mineral substance formed in the seams of rocks below the surface of the earth. From the depths of the earth comes a piece of the sky.
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