Turquoise is a semi-precious mineral prized globally for its distinct color, highly valued across numerous cultures for thousands of years. The name is thought to derive from the French word for “Turkish,” reflecting the trade route through which the stone first arrived in Europe from Central Asia. This blue-to-green gemstone holds deep historical significance, used in adornment by ancient Egyptians, Mesoamericans, and Persians.
The Geological Formation of Turquoise
Turquoise is classified as a secondary mineral, meaning it forms through the alteration of pre-existing minerals by water near the Earth’s surface, not directly from magma. Chemically, it is a hydrated copper aluminum phosphate.
The formation requires acidic, copper-rich groundwater to percolate through aluminous rock formations, typically in arid or semi-arid climates. Copper provides the characteristic blue hue, while iron impurities can result in greener shades. The water leaches copper from primary copper sulfides or carbonates and combines it with aluminum and phosphorus from minerals like feldspar and apatite.
This slow chemical reaction occurs in fractures, cavities, and veins within the host rock, often at relatively shallow depths of less than 65 feet. Turquoise forms as a cryptocrystalline aggregate of tiny crystals, making it inherently porous and relatively soft, generally rating between 5 and 6 on the Mohs scale of hardness. The surrounding host rock, or matrix, is often left embedded, creating the unique brown or black webbing patterns seen in many specimens.
Major Global Mining Sources
The geographical origin of turquoise is closely tied to the distribution of copper deposits in dry, weathered regions. For millennia, the standard of quality was set by “Old World” mines, particularly those in the Khorasan Province of Iran. The Nishapur mines, active for over 2,000 years, are famous for producing the intense, flawless sky-blue color known as “Persian Blue.”
The finest Iranian turquoise, sometimes called Ajami, is highly valued for its dense structure and lack of matrix. Specimens with attractive veining, known as Shajari, are also prized. In the “New World,” the American Southwest holds significant deposits, with New Mexico, Arizona, and Nevada being the primary sources. These deposits are often associated with large copper mining operations, where turquoise is sometimes recovered as a byproduct.
The Cerrillos mining district in New Mexico is recognized as the site of the largest prehistoric mining activity in North America, worked by the Pueblo people for over a thousand years. Arizona’s Kingman mine is one of the oldest and largest producers, yielding stones in a wide range of blue and green shades, frequently featuring black spiderweb matrix patterns. The famed Sleeping Beauty mine, also in Arizona, was known for producing a consistent, matrix-free, robin’s-egg blue color before its closure in 2012, making its output highly collectible. China, specifically the Hubei province, is a significant contemporary source, producing material that often exhibits distinctive green-blue colors and dark matrix.
Identifying Authenticity and Treatments
The inherent porosity and relative softness of natural turquoise mean that much commercial material undergoes treatment to improve durability and appearance. The most common process is stabilization, which involves infusing porous, lower-grade turquoise with clear epoxy or plastic resins under pressure. This fills microscopic voids, increasing the stone’s hardness and preventing color change due to absorption of skin oils or chemicals.
Another form of alteration is reconstituted turquoise, made by crushing soft, chalky turquoise fragments into a powder. This powder is then mixed with a plastic binder and often dye, molded into blocks, and cut into shapes. While it contains genuine turquoise material, this product is chemically and structurally altered from the natural stone.
The market also contains various simulants, which are materials that contain no actual turquoise but are dyed to imitate its color. Frequent examples include dyed howlite and magnesite, both softer, more porous white minerals that readily absorb blue dye. Distinguishing between natural, stabilized, reconstituted, and simulated turquoise is crucial for consumers, as the value decreases significantly with each level of alteration away from a naturally hard, untreated stone.