Turquoise is an opaque, blue-to-green mineral prized as a gemstone for its unique and vibrant color, with the finest grades being quite rare. Chemically, it is a hydrous phosphate of copper and aluminum. This striking stone has been treasured for millennia across various cultures, from ancient Egyptians who used it in jewelry and adornments to Native American tribes in the Southwest who consider it a protective stone. Its scarcity and historical significance mean that knowing its geographical origin can be important, as source often influences the stone’s color, quality, and market value.
The Geological Requirements for Turquoise Formation
The formation of turquoise is a slow, secondary geological process that occurs relatively close to the Earth’s surface, unlike many other gemstones. It requires a specific cocktail of chemical elements and a unique environmental setting to precipitate from water. The blue-to-green hues of the stone are directly linked to the presence of copper, which provides the blue coloration, while iron impurities often introduce a greenish tint.
The necessary ingredients include a source of copper, often from pre-existing copper sulfide deposits, a source of aluminum, and a source of phosphorus. These elements must be dissolved by acidic water, typically meteoric water from rain or snowmelt, that percolates through the rock. The environment must be arid or semi-arid, as this climate promotes the slow evaporation and chemical concentration necessary for the mineral to precipitate into veins, seams, or nodules within the host rock.
This process, known as supergene alteration, restricts turquoise deposits to the oxidized zones of copper deposits, generally at shallow depths. The resulting stone is microcrystalline and porous, with a Mohs hardness ranging between 5 and 6. This specific combination of copper-rich host rock and arid climate explains why major turquoise sources are geographically limited.
Historic and Global Sources
The history of turquoise mining stretches back over 4,000 years, with the oldest known sources found outside of North America. For centuries, the global standard for quality was set by the deposits in what is now Iran. The Nishapur region in the Khorasan province has been a significant source for at least two millennia, producing the coveted “Persian blue” stone.
Stones from Nishapur are historically renowned for their intense, uniform sky-blue color and a notable lack of matrix, which refers to the remnant host rock veining. This material was a major trade commodity, traveling along ancient routes. Another ancient source is the Sinai Peninsula in Egypt, where mining has been documented since at least the First Dynasty, and the Egyptians used the stone extensively for jewelry and tomb adornments.
In more recent times, China has emerged as a major global supplier, with significant deposits located in provinces like Hubei. Chinese turquoise often exhibits a color palette ranging from greenish-blue to light green, a result of lower copper content and higher iron or aluminum. This material is frequently characterized by black or brown matrix veining, distinguishing it visually from the classic, clean Persian-type stones.
Major Deposits in North America
The Southwestern United States contains extensive and diverse turquoise deposits, making it highly relevant to the modern market. The states of Arizona, Nevada, New Mexico, and Colorado are the primary producers, often yielding stones as a byproduct of large-scale copper mining operations. Each state, and often individual mines within them, produces material with distinct visual characteristics based on the local geology and mineral impurities.
Arizona Deposits
Arizona is home to some of the most famous mines, including the Kingman mine, known for its bright, sky-blue color, which is a common commercial standard. The now-closed Sleeping Beauty mine, located near Globe, was celebrated for producing a pure, light blue stone with little to no matrix, making it visually comparable to the fine Persian material. Another highly collectible Arizona stone is Bisbee turquoise, known for its deep blue color and distinctive dark chocolate-brown to black matrix patterns.
Nevada Deposits
Nevada is particularly notable for its wide variety of turquoise, with over 100 historical mines producing stones that range from deep blues to vibrant greens. The color variation in Nevada turquoise is often due to the substitution of aluminum with iron, which creates the green tones, or sometimes zinc, which results in bright green to yellow-green shades. Famous Nevada mines like Royston and Number Eight are known for their intricate spiderweb matrix patterns, where the host rock forms a fine network of veins throughout the blue or green stone.
Sourcing and Identifying Genuine Stone
When looking to acquire turquoise, seeking out reputable gem dealers, specialized jewelry stores, or established gem shows is the most reliable approach. The origin of the stone is a factor in its valuation, as material from historically famous mines or those that produce a rare, uniform color commands a higher price. Unless a stone is accompanied by documentation tracing its path from the mine, a definitive identification of its exact source can be challenging, even for experts, due to the visual overlap between deposits.
A key factor in initial assessment is color saturation, which is largely controlled by the copper content; a richer blue suggests a higher concentration of this element. High-quality turquoise is naturally hard, and its surface should feel smooth and dense. A stone that is perfectly uniform in color and overly bright should be viewed with suspicion, as natural turquoise often displays subtle variations in hue and may contain a matrix. Buyers should focus on the quality of the stone’s appearance and the reputation of the seller rather than relying solely on a geographical name.