Turquoise is an opaque, blue-to-green mineral valued as a gemstone and ornamental stone for thousands of years, from Ancient Egypt to the Aztecs. The name “turquoise” is thought to originate from the French word for “Turkish,” as the mineral arrived in Europe from Persia (modern-day Iran) via trade routes through Turkey. Understanding its physical and chemical makeup helps differentiate this unique gem from the many imitations available today.
The Core Chemical Recipe
Turquoise is chemically classified as a hydrous phosphate of copper and aluminum. Its precise chemical formula is CuAl6(PO4)4(OH)8 · 4H2O, indicating the presence of copper, aluminum, phosphorus, oxygen, and water molecules.
Copper is the most significant element, responsible for the vibrant blue hues of the stone. Aluminum and the phosphate group form the structural base of the mineral. The inclusion of water molecules (4H2O) classifies it as a hydrous mineral.
Geological Formation and Environment
Turquoise is a secondary mineral, meaning it forms through the alteration of pre-existing minerals rather than directly from molten rock. This process typically occurs in arid or semi-arid environments. It begins when meteoric water, such as rainwater, percolates through the Earth’s crust.
This acidic, oxygenated water dissolves copper, aluminum, and phosphate from surrounding minerals. Copper often originates from nearby copper sulfide deposits, while aluminum may come from feldspar and phosphate from minerals like apatite. The resulting solution precipitates turquoise in veins, nodules, or masses within host rocks like limestone, sandstone, or altered volcanic rocks. Mineralization is usually limited to shallow depths, often less than 20 meters below the surface.
Color, Matrix, and Physical Characteristics
The color of turquoise spans from vibrant sky-blue to deep yellowish-green, linked directly to trace elements present during formation. Pure blue is attributed to copper content. A shift toward green hues is caused by iron replacing some of the aluminum in the mineral’s chemical structure.
Turquoise is nearly always cryptocrystalline, meaning its crystals are too small to be seen without magnification. It typically forms in massive or nodular habits. Its hardness on the Mohs scale ranges from 5 to 6, comparable to window glass. Denser, less porous stones are harder, while softer, chalkier material is more porous.
Many specimens feature a pattern of darker material running through the stone, known as the matrix. This matrix is composed of remnants of the original host rock, such as limonite, sandstone, or iron oxide. When the matrix forms thin, interconnected lines, the stone is described as “spider-web” turquoise.
Distinguishing Natural from Treated and Fake Turquoise
Due to demand and variable quality, much of the turquoise found in the market is treated or imitated. The most common treatment is stabilization, where porous, softer turquoise is infused with a plastic resin or epoxy. This process enhances the stone’s color, improves durability, and makes it harder for use in jewelry.
Another common material is reconstituted turquoise, made by crushing low-grade turquoise powder and mixing it with dyes and resin to form a solid block. Completely fake turquoise, or simulants, contain no actual turquoise. The most common simulant is dyed howlite or magnesite, which are naturally white minerals dyed blue to resemble the turquoise matrix.
A quick test for dyed imitations involves rubbing a small area with a cotton swab soaked in acetone. If the cotton turns blue, it indicates the color is a surface dye, as the dye rarely penetrates deep into the mineral. Natural, untreated turquoise is opaque, relatively hard, and will not yield color to a solvent.