Turquoise is an opaque mineral prized by cultures around the world for its distinctive coloration. Chemically, it is a hydrated phosphate of copper and aluminum, forming primarily in arid regions near copper deposits. Ancient peoples, including the Egyptians and Persians, valued turquoise as early as 6,000 B.C. for its ornamental beauty. The mineral’s ability to present a color between blue and green is rare, establishing it as a highly sought-after gemstone.
The Spectrum of Turquoise Hues
The color of turquoise is not uniform, presenting a wide spectrum that defines its quality and desirability. The most prized shade is a clear, medium-toned, intense sky blue, often referred to as “Persian Blue” or “robin egg blue.” This vibrant blue sets the standard against which other turquoise is judged, representing the stone’s ideal appearance without significant contaminants.
Moving away from pure blue, the color transitions through various shades of bluish-green and greenish-blue. These colors are common and widely appreciated, offering a deeper or more muted tone compared to the intense sky blue. The spectrum extends further into apple green, dull green, and even yellowish-green hues, which are typically less valuable but offer a different aesthetic.
Chemical Elements Determining Color
The fundamental color of the turquoise mineral is determined by its precise chemical composition and the presence of specific trace elements. The element copper is the primary source of the stone’s blue coloration, with copper ions within the crystal lattice absorbing light in the red spectrum and reflecting the blue. A higher concentration of copper is directly responsible for the coveted, intensely blue shades.
The shift toward green hues is caused by the presence of iron, which enters the turquoise structure by substituting for aluminum. Iron introduces a subtle greenish undertone, and an increasing ratio of iron to copper will push the stone’s color from blue-green to a distinct green. This formation occurs when acidic aqueous solutions, containing copper, aluminum, and phosphate, percolate through fissures in host rocks. The resulting mineral takes on the color dictated by the mix of available trace elements, with the ratio of copper to iron being the final determinant of the exact shade.
Understanding the Matrix and Veining
While copper and iron content determines the mineral’s base color, the overall appearance of a finished turquoise stone is often defined by the matrix and veining. The matrix is the remnant of the host rock—the material surrounding the turquoise mineral during its formation—that remains visible within the stone. This matrix material can be varied, appearing in colors like black, brown, yellowish-brown, and rust-red, offering a stark contrast to the blue or green mineral.
The most famous matrix pattern is the “spiderweb,” an intricate network of thin, dark veinlets that interweave across the surface of the turquoise. This pattern is created when host rock fragments remain evenly spaced throughout the turquoise. The density and color of the matrix can greatly enhance the stone’s aesthetic value.