What Element Makes Turquoise Its Color?
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Turquoise is mineral that is pale blue, greenish-blue, bluish-green or green. The blue comes from copper and the green comes from the presence of iron in the stone. Since the iron is in only trace amounts -- if it is there at all -- commonly, the stone is largely blue. The more iron there is, the greener the tint to the stone and the less valuable it is. The color of turquoise is so characteristic that it was named after the stone.
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Spiderweb turquoise is a kind of turquoise that, besides having its characteristic color, also has lines running through it, giving the surface a crackled look. The lines are red to black and come from iron oxide filling in cracks in the turquoise stone. Though "spiderweb" seems to imply the lines will form a pattern, they don't. In some places, spiderweb turquoise is more sought after than pure blue turquoise, making it the most valuable.
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The chemical formula for turquoise is CuAl₆(PO₄)₄(OH)₈∙5 H₂O. The Cu in the formula is the symbol for copper, Al is the symbol for aluminum and the PO₄ part of the formula indicates phosphate, which is a salt made of phosphoric acid. Thus, turquoise is a hydrated (H₂O is the formula for water) copper aluminum phosphate. It is formed as water beneath the surface of the Earth comes in contact with hot rock. The minerals in the waters interact with those of the rock, pulling the elements that make up turquoise. When the fluids cool, the atomic ingredients arrange themselves into turquoise. Iron (Fe) can take the place of aluminum in that arrangement, shifting the color toward the green. If the iron completely replaces aluminum, the product isn't turquoise, it's chalcostibite.
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The hot rock present as part of the beginnings of turquoise comes from the movement of magma intruding into rock. The process of weathering means the turquoise ends up exposed or at shallow depths where it can be found, running through the rock in small veins or in stringers, usually in arid places. (Stringers are more like filaments than veins in rocks.) The Southwestern United States is the source of most turquoise. The finest colored stones have been traditionally mined in Iran to the point that specimens from newer sources with high quality stones might be called Persian turquoise regardless of where they were mined. Turquoise was the first gem to be mined, with mines going back to more than 5,000 years ago.
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