With its rich variation of blue-green hues, turquoise is one of the most recognizable minerals in the world. Geologically speaking, turquoise is composed of phosphate and a mix of iron, copper and other metals. It forms naturally through weathering and oxidation, and is found in arid regions around the world, particularly in copper-rich soils. It is this copper that contributes the blue color, while nearby iron deposits add green in varying quantities. There are significant turquoise deposits in the American Southwest and in the deserts of northeastern Iran.
Like other minerals, turquoise varies greatly in quality. Mining has depleted much of the more valuable grades of the mineral located close to the surface. Chalk turquoise is a low-quality variety, found deeper in the soil. As a result, it is much softer and more brittle. On the Mohs hardness scale, chalk turquoise ranges between 2-4, lower than the 6 of higher grades. It lacks the blue shade that defines the mineral. Instead, chalk turquoise is pale blue or white. It may or may not have significant matrixing, the brown or black flecks often associated with Native jewelry of the southwest. "Simulated" turquoise is often touted as being chalk turquoise, when in fact it is just dyed Magnesite or another white stone that absorbs dye easily.
Turquoise has been used in jewelry for more than three thousand years. Asians believed that it was blessed by Buddha, while in Arab cultures the gem was worn to steer away evil. Native Americans also thought turquoise to be lucky, and made much of their jewelry from the mineral to bring good fortune. Chalk turquoise cannot be used to make jewelry because it is too fragile, but there are ways around this weakness.
Jewelers may stabilize the turquoise by grinding it into a powder and allowing it to soak in clear acrylic; once it has hardened, it can then be dyed to enhance the blue color, or dyed another color altogether. While many people would view chalk turquoise as a cheap knockoff to the "real thing," there are advantages to using it as jewelry. For one, it resists chemicals like chlorine; in addition, it will not be affected by the oil in the wearer's skin, which can change the color of natural turquoise over time.