Yes, turquoise is a gemstone, classified as an opaque mineral that has been prized for millennia for its unique color palette. This material is widely recognized for its distinct hues, which range from sky-blue to various shades of green. It has been a treasured ornamental material across diverse cultures, from ancient Egypt to the American Southwest. The unique aesthetic qualities of turquoise make it a popular choice for jewelry and decorative objects today.
Defining Turquoise as a Gemstone
Turquoise is scientifically classified as a hydrous phosphate of copper and aluminum. Its status as a gemstone is confirmed because it is a naturally occurring mineral that is cut, polished, and used for adornment. This composition places it within the phosphate mineral group, distinct from silicate-based gems like quartz.
The stone’s specific coloration results from trace elements present during its formation. Copper causes the characteristic blue color within the crystal structure. A shift toward green or yellowish-green hues occurs when iron or zinc substitutes for the aluminum, creating the full spectrum of colors seen in natural turquoise.
Geological Formation and Primary Mining Sources
Turquoise is categorized as a secondary mineral because it does not form deep within the Earth’s crust. Instead, it forms near the surface through the percolation of acidic, copper-rich water through host rock. This solution reacts with aluminum and phosphorus-bearing minerals in arid environments. The process results in the precipitation of turquoise within veins, seams, and nodules, typically within 20 meters of the surface.
Historically, significant deposits were found in the Nishapur district of Iran, which set the standard for the finest color. Early mining also occurred in the Sinai Peninsula, with operations dating back to the fourth millennium BC. Today, a major source of the world’s supply comes from the American Southwest, particularly Arizona and Nevada.
Evaluating Quality: Color, Hardness, and Matrix
The value of a turquoise stone is evaluated based on three primary factors: color, hardness, and the presence of the matrix.
Color
The most highly valued color is an even, intense medium blue, often described in the trade as “Persian blue” or “robin’s egg blue.” While pure blue is traditionally most sought after, the presence of green tones generally lowers the price. However, some collectors now specifically seek out rare lime-green or avocado hues.
Hardness
Turquoise is relatively soft for a gemstone, measuring between 5 and 6 on the Mohs scale of mineral hardness. This softness and the stone’s naturally porous structure make it susceptible to scratching and can allow it to absorb oils or chemicals, which may cause a permanent color change. Higher-grade natural turquoise tends to be less porous and therefore harder, which enhances its durability and polish.
Matrix
The matrix refers to the remnants of the host rock—such as limonite or ironstone—that appear as veins or patches within the turquoise. The presence of a matrix can significantly affect value, which is dependent on its aesthetic appearance. A fine, delicate network of dark veining, known as a “spiderweb matrix,” is often highly sought after by collectors and can increase the value. Conversely, a coarse or unattractive matrix can detract from the stone’s overall appeal and commercial worth.
Treatments, Enhancements, and Imitations
Due to the natural softness and porosity of most mined material, turquoise often undergoes treatment or enhancement.
Treatments and Enhancements
The most common process is stabilization, which involves impregnating the stone with an epoxy or plastic resin under pressure and heat. This treatment seals the pores, increases the stone’s hardness and durability, and prevents color change. Other enhancements include dyeing to deepen the hue or darken the matrix for contrast.
Imitations
Low-quality turquoise fragments are often ground into a powder, mixed with a binding agent and dye, and pressed into slabs to create “reconstituted” turquoise. This material contains real turquoise but is not a solid natural stone. Consumers should also be aware of outright imitations, which contain no actual turquoise material.
Common imitations include:
- Dyed howlite or magnesite, which are cheaper, porous minerals colored to mimic turquoise.
- “Block turquoise,” which is a mixture of plastic and other materials.
- Materials sometimes containing iron pyrite added to simulate a matrix, and have no inherent gemstone value.