Turquoise is quite possibly one of the oldest known gemstones. Pharaohs, early Native Americans, and the Aztecs Kings have all used and prized turquoise. The word turquoise has even become synonymous with that particular color of blue or colors of blue that are close to it. However, only the blue colored turquoise is so prized and considered of any value. The color and therefore the value of turquoise is directly related to how it forms and what its composition is.
Formation of turquoise
Turquoise is only found near deposits of copper. This is due to copper’s importance in its formation. In addition to the copper, turquoise requires the presence of three other materials. These are aluminum ions from feldspar which is typically present near copper deposits, trace amounts of water, and phosphorus containing minerals such as apatite or phosphorus containing compounds like calcium phosphate.
Unlike other minerals or rocks, turquoise does not form deep within the earths crust or as a result of heat and pressure like rubies or granite. It is instead created through weathering and chemical reaction between the previously listed constituents (copper, aluminum, phosphorus, and water). As water percolates into the ground over time or as the result of groundwater, it may come into contact with aluminum containing rocks/material, phosphorous, and the presence of copper deposits. These are the necessary conditions for the formation of turquoise. It is a secondary mineral that forms in the copper deposits as a result of the chemical changes that occur when all these constituents are present. The water will bring the aluminum ions and phosphate to the copper which will begin to change and form turquoise. This Formation typically occurs in desert, arid, or semiarid environments. Many places around the world produce turquoise such as parts of the United States, Iran, and Egypt.
Chemical composition
The composition of turquoise is a hydrated phosphate that is combined with copper and aluminum. The formula for turquoise is CuAl6(PO4)4(OH)8(4(H2O)). It is quite long formula and includes every constituent that was mentioned before. This formula is however not always typical for turquoise but it is for the blue turquoise. The present of iron in the ground will change the color of the turquoise and alter this formula throughout some or all of the turquoise.
Coloring
Iron-free turquoise is the typical sky-blue that is associated with the word turquoise and that is the most valuable. Iron that is present near forming turquoise can substitute itself for aluminum ions. This results in a variation of color. The more iron present in the turquoise, the more the color changes from blue to green. Should the iron completely replace the aluminum and then it is no longer considered turquoise and is called chalcosiderite. Since the replacement happens gradually, a whole spectrum of colors between the pure turquoise blue and the chalcosiderite green is possible. Unfortunately, as the color becomes greener, the value of the turquoise diminishes.
Treated turquoise
Because demand for turquoise became so great, another industry emerged that formed around the manufacture of synthetic turquoise. Only a portion of all the available turquoise was actually usable for jewelry as it was of lesser quality or poorly colored that would not have normally been of any value. To change the materials, hot acrylic resins are pressure-impregnated into these lesser types of turquoise. The addition of the resin serves to better the color, durability, and even the hardness of the poorer quality turquoise. The result is that more turquoise is available for use in jewelry. This synthetic material must be properly labeled as ‘treated’ or ‘manufactured’ but has found a place in the market for those that really want the look of turquoise.
