Turquoise-raw and polished cabachons

Turquoise-raw and polished cabachons

     Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminum. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue.

     Turquoise was among the first gems to be mined by man, and while many historic sites have been depleted, some are still worked to this day. These are all small-scale, often seasonal operations, owing to the limited scope and remoteness of the deposits. Most are worked by hand with little or no mechanization. However, turquoise is often recovered as a byproduct of large-scale copper mining operations, especially in the United States. The turquoise occurs as vein or seam fillings, and as compact nuggets; these are mostly small in size. The Southwest US is a significant source of turquoise. Arizona is currently the most important producer of turquoise by value.

     The mother rock or matrix in which turquoise is found can often be seen as splotches or a network of brown or black veins running through the stone in a netted pattern. This veining may add value to the stone if the result is complementary, but such a result is uncommon. Such material is sometimes described as "spiderweb matrix"; it is most valued in the Southwest US and Far East, but is not highly appreciated in the Near East where unblemished and vein-free material is ideal. Uniformity of color is desired, and in finished pieces the quality of workmanship is also a factor; this includes the quality of the polish and the symmetry of the stone. Calibrated stones—that is, stones adhering to standard jewelry setting measurements—may also be more sought after. Like coral and other opaque gems, turquoise is commonly sold at a price according to its physical size in millimeters rather than weight. 


tanna@tannathornburg.com © Tanna Thornburg 2014