Turquoise is an opaque mineral defined as a hydrated phosphate of copper and aluminum. This chemical structure, formed by the interaction of copper-rich groundwater with aluminous rock, gives the stone its characteristic blue-to-green coloration. The vast majority of turquoise available commercially has been chemically treated, and many items sold as turquoise are complete fakes. Learning to discern the genuine, unaltered stone from common treatments and simulants is necessary, as truly raw turquoise is rare and highly prized.
Inherent Physical Characteristics of Raw Turquoise
Raw, untreated turquoise establishes authenticity through its natural physical characteristics. The stone’s color is determined by its composition: copper causes sky-blue hues, while iron content leads to greener tones. Natural color variations are not uniform, often showing a gradual shift between shades within a single specimen. When unpolished, the surface exhibits a waxy to subvitreous luster, appearing duller than highly processed counterparts.
The hardness of turquoise is moderate, typically registering between 5 and 6 on the Mohs scale. This relatively low hardness means the stone is naturally porous and susceptible to damage and discoloration. Raw turquoise is a cryptocrystalline aggregate, composed of microscopic crystals, which contributes to its opaque nature and sometimes chalky texture.
A significant feature of many raw specimens is the matrix, the remnant of the surrounding host rock, such as limonite or sandstone. This matrix appears as dark brown, black, or tan veining running through the blue material. When this host rock forms thin, interconnected lines, it is known as “spiderweb” turquoise, a pattern highly valued as an indicator of natural origin. The presence and irregularity of this veining is a strong clue to the stone’s genuine, unaltered state.
Identifying Common Treatments and Simulants
Because raw turquoise is often too soft and porous for jewelry use, much of the market consists of treated material designed to increase durability and enhance color. Stabilization is the most common treatment, involving impregnating the stone with polymers, plastic, or epoxy. This process fills the pores, creating a harder, less absorbent material that often displays an unnaturally uniform color and a distinct plastic-like sheen.
Another prevalent type is reconstituted or block turquoise, which is a composite material rather than a single natural stone. This is made by grinding down small, low-grade fragments or powdered turquoise, mixing it with a binder such as resin, and then pressing it into blocks and dyeing it. Visually, this material often exhibits a repeating, unnatural pattern in the matrix or a clear separation line where the dye failed to fully penetrate the binding agent.
When a stone is not turquoise at all, it is called a simulant; dyed Howlite and Magnesite are the most frequent imposters. Both are naturally white minerals with dark veining that are easily dyed blue to mimic turquoise. The veining in dyed Howlite or Magnesite can sometimes appear too uniform or painted, and the color is usually concentrated solely on the surface. Plastic or glass simulants also exist. These typically look too perfect in color and lack the subtle variations inherent in a natural mineral.
Simple At-Home Verification Tests
Several non-destructive tests can be performed at home to verify a stone’s authenticity or treatment status. The simplest is the Acetone Test, which targets surface dyes used in simulants like Howlite or in dyed stabilized turquoise. To perform this, a cotton swab dampened with acetone or nail polish remover should be rubbed on a small, inconspicuous area of the stone. If the swab picks up a blue or green color, the stone has been dyed.
Another quick check is the touch and weight test, which provides an initial assessment of the material. Real, dense turquoise feels noticeably cool to the touch and possesses a satisfying heft or specific gravity for its size. Conversely, plastic, resin-heavy simulants, or highly porous fakes often feel warmer and lighter.
A cautious Scratch Test can be employed in a discreet location, such as the bottom edge of a cabochon. Since turquoise has a moderate Mohs hardness of 5 to 6, it should not scratch easily with a fingernail, which might indicate a softer material like chalky Howlite or soft plastic. If the stone is scratched, a white or light-colored powder beneath the surface indicates the blue color is only a surface dye.
The most definitive, though potentially destructive, method is the Hot Needle Test, which identifies plastic or resin binders. A needle or metal scribe is heated until red-hot and then carefully pressed against an unseen spot on the stone. If the stone is plastic or contains a heavy resin binder, the needle will cause it to melt, gouge, or emit an odor of burning plastic. Genuine turquoise, being a mineral, will not melt and remains unaffected by the brief application of heat.