The Kingman turquoise mine, operated by the Colbaugh family in Arizona, is the last remaining commercially producing mine in the United States. Evidence suggests turquoise was mined in the surrounding areas dating as far back as 600 AD. Tools like stone hammers were found in the ancient trenches and tunnels running through the hills in 1962.
Colbaugh Processing operates the mine and specializes in stabilized turquoise, whereby epoxy is infused into the stone which deepens the color and makes the chalky material suitable for cutting and polishing. Their secret process is heavily used to treat a variety of medium to low-grade turquoise produced at Kingman and other mines in the US and abroad.
Much of the turquoise has a white matrix that is often dyed black for better contrast. The quality natural Kingman turquoise produced at this mine is generally medium blue with a “water web” matrix that easily distinguishes it from another turquoise. The best turquoise is a lovely blue with a black web matrix. The Kingman mine also produces a variety of turquoise ranging from pale green to blue with a red, brown, or yellow matrix.
Leonard Hardy operated the mine in the 1970s and at that time Kingman was producing almost half of the world’s supply of turquoise. Over 80,000 pounds were produced in 1973 alone, according to an estimate from Leonard Hardy. So many thousands of pounds, both treated and natural, have been produced at the Kingman mine over the years that any time a turquoise is not easily identified or lacks provenance, there is a good chance it is Kingman.
Today, even the quality natural turquoise from Kingman has become rare in the wake of rising demand for good quality American turquoise. [Source]