Turquoise is a popular opaque gemstone, appreciated for its distinctive sky-blue to green hues. Its classification—whether it is a crystal, a mineral, or a rock—often leads to confusion. The definitive answer lies in the material’s microscopic structure, which helps categorize turquoise precisely within the family of natural solids. Understanding this requires a clear definition of what a crystal truly is in the scientific context.
Understanding the Definition of a Crystal
A crystal is defined by an internal atomic structure that is highly ordered and repeating in a three-dimensional pattern, known as a crystal lattice. This lattice is made of a consistent, small unit cell that translates repetitively along its axes, giving the entire solid a uniform structure. This precise arrangement dictates many physical characteristics of the material, including properties like cleavage and hardness.
Minerals are naturally occurring, inorganic solids with a specific chemical composition and a characteristic internal structure. While all crystals are minerals, not all minerals form large, visibly defined crystals. Common examples of true, single-crystal minerals include quartz and halite (table salt). The existence of this underlying, ordered lattice is the fundamental criterion for a substance to be considered crystalline.
The Chemical Makeup of Turquoise
Turquoise is chemically classified as a mineral species: a hydrated phosphate of copper and aluminum. Its chemical composition is \(\text{CuAl}_6(\text{PO}_4)_4(\text{OH})_8\cdot4\text{H}_2\text{O}\). The presence of copper is responsible for the characteristic blue color, while iron impurities can shift the color toward green.
The mineral forms as a secondary deposit near the surface, rather than deep within the Earth under intense heat and pressure. It develops when acidic, copper-rich groundwater percolates through aluminous rock in arid regions. This solution interacts with aluminum-bearing minerals and phosphate, resulting in the precipitation of turquoise in veins, nodules, or crusts within the host rock.
Why Turquoise is Classified as Cryptocrystalline
Turquoise is scientifically classified as a cryptocrystalline mineral. The term “cryptocrystalline” literally means “hidden crystal,” indicating that the mineral is composed of crystals too small to be seen without a powerful microscope. X-ray diffraction confirms that turquoise possesses the necessary internal atomic lattice—specifically a triclinic structure—but it rarely forms a single, macroscopic crystal.
The turquoise used in jewelry is massive, meaning it exists as an aggregate of countless, minute, interlocking crystals. These tiny grains are packed together to form the opaque, solid material. This structure gives turquoise a waxy luster and prevents it from being classified as a single, visible crystal.