Turquoise is an opaque mineral valued as a gemstone for thousands of years, prized for its distinctive range of blue and green colors. Historically, this striking stone has been used across ancient civilizations, including in Egyptian burials and by various cultures in the Americas and Asia. Turquoise is a relatively soft, porous mineral, which influences both its geological formation and its use in jewelry.
The Mineralogical Identity
Turquoise is classified as a hydrous phosphate of copper and aluminum, placing it within the larger phosphate mineral group. The presence of copper is responsible for the stone’s signature blue coloration, while iron inclusion leads to greenish tints.
The spectrum of color, ranging from sky blue to green, is directly related to the inclusion of other elements during formation. When iron replaces some of the aluminum in the stone’s structure, the turquoise takes on a greenish tint. As a cryptocrystalline mineral, turquoise is composed of a dense aggregate of microscopic crystals, making single, macroscopic crystals extremely rare. This minute structure contributes to its opacity and characteristic waxy or subvitreous luster.
Geological Formation and Environment
Turquoise is a secondary mineral, meaning it does not form as a primary crystal deep within the Earth under intense heat and pressure. Its formation is the result of a slow chemical process that occurs relatively close to the surface. This process requires the presence of copper, aluminum, and phosphorus, typically occurring in arid or semiarid environments.
The formation begins when acidic, aqueous solutions, often rainwater, percolate downward through host rock containing these necessary elements. The water dissolves copper and aluminum from pre-existing minerals, while also incorporating phosphorus. The resulting solution then deposits the new mineral in existing fissures, seams, and cavities of the host rock, solidifying into turquoise. This deposition is a cold-weathering process, distinct from the high-temperature formation of many other gems.
Physical Characteristics and Quality Factors
The hardness of turquoise, typically measuring between 5 and 6 on the Mohs scale, is a significant factor in determining its quality and durability. Due to its low hardness and porous nature, turquoise is susceptible to scratching and can absorb oils and liquids, which may alter its color over time. The most highly valued turquoise exhibits a strong, consistent color, with the intense sky-blue or “robin’s egg” blue being the traditional benchmark for quality.
A common feature of turquoise is the presence of a matrix, which is the remnant of the surrounding host rock interlaced within the stone. This matrix often appears as a dark, spider-web pattern, with colors ranging from brown to black. While a clean, solid color is often sought after, a fine, evenly distributed “spiderweb” matrix can also enhance the stone’s appeal and value. The best material is naturally hard and dense, allowing it to take a high polish.
Treatments and Imitations
Because a majority of mined turquoise is too soft or porous for jewelry use, various treatments are commonly applied to improve its durability and appearance. One of the most prevalent is stabilization, where low-grade, porous material is pressure-treated with a clear epoxy or plastic polymer. This process hardens the stone, enhances the color, and prevents the material from absorbing substances that cause discoloration.
Another form of alteration is reconstitution, which involves grinding small pieces or powder of turquoise and mixing the material with a binding resin, often with added dyes, to form a solid block. This block can then be cut and polished into cabochons or beads. These treated materials are distinct from true imitations, which contain no natural turquoise.
Common imitations include dyed stones like howlite or magnesite, which are stained to match the blue or green hue of natural turquoise. Synthetic turquoise, such as Gilson turquoise, is a lab-created material that replicates the chemical and physical properties of the natural stone. These imitations and treatments make the supply of softer turquoise suitable for the market, but they must be disclosed to the consumer.