What is turquoise? Nishapur, Arizona and the treatment question that matters

Chemistry, colour, and the porosity problem

Turquoise forms as a secondary mineral in the weathered zones above copper ore deposits, where copper-bearing groundwater percolates through aluminium-rich rocks and precipitates the copper aluminium phosphate in cracks and porous zones. The colour is produced by Cu2+ (cupric copper): pure copper gives the vivid sky blue of finest Persian turquoise; iron substitution shifts the colour toward green. The spider web pattern seen in many turquoise cabochons is the matrix (host rock) visible through the turquoise vein system, not a defect in the turquoise itself (GIA Gem Reference Guide, 2006, pp. 106-107; Wise, 2016, pp. 343-346).

The porous microstructure of most natural turquoise is its defining quality challenge. Porosity means turquoise absorbs oils, perspiration, cosmetics, and cleaning agents, causing colour change over time. This absorption also means turquoise readily accepts dyes and stabilising resins, which is why the treatment grade system is so central to turquoise value (GIA; Wise, 2016).

Nishapur, Iran: the world's finest turquoise

The Nishapur deposit in Khorasan Province, near the city of Mashhad in northeastern Iran, is the benchmark fine source for turquoise. The specific geology of Nishapur, where turquoise veins occur in altered volcanics with specific aluminium and copper chemistry, produces the vivid sky blue colour without the greenish cast that characterises most other sources. The finest Nishapur grade is called "Abi" (sky blue): a vivid, saturated pure blue at high saturation and relatively hard consistency for turquoise, making it less porous and more durable than softer varieties (GIA; Wise, 2016, pp. 346-349).

Nishapur also produces "Shajari" turquoise with black spiderweb matrix, which has its own collector following. Production continues but at declining volumes from the ancient deposits.

Arizona and the American turquoise tradition

Arizona and Nevada produce significant quantities of turquoise across a wide quality range. The Sleeping Beauty mine (Globe, Arizona) produced a distinctive sky blue turquoise without matrix, praised for its clean colour. The mine ceased turquoise production in 2012 and converted to copper production, making Sleeping Beauty turquoise from the mine's active years increasingly collectable. The Kingman mine (Kingman, Arizona), Morenci, Bisbee, and other Arizona deposits each have distinctive colour and matrix characters recognised by collectors of American turquoise (GIA; Wise, 2016, pp. 349-352).

Most Arizona and Nevada turquoise is stabilised (see treatment grades below) before commercial sale because the natural material is typically too soft and porous to hold a good polish without treatment. The American Southwest turquoise tradition, particularly its significance in Navajo, Zuni, and Hopi jewellery, involves stabilised turquoise in virtually all commercial production since the 1970s (GIA; Wise, 2016).

The turquoise treatment grading system

The turquoise trade uses a letter grading system that is the most important quality framework for the material:

Turquoise treatment grades A through D. Grade A (natural/untreated) commands the highest prices; Grade D (dyed howlite, plastic, or glass) must be disclosed as simulated. Most commercial turquoise in the global market is Grade C (stabilised) or Grade D. Source: GIA; AGTA; Wise (2016).

Turquoise simulants: what is sold that is not turquoise

The range of materials sold as turquoise is wider than for almost any other gem species. The most common simulants:

Howlite: A white calcium borosilicate mineral that accepts blue-green dye readily, producing an appearance close to turquoise. Dyed howlite is the most common turquoise simulant globally. The dye is not stable and fades with UV exposure and washing. Howlite is harder than turquoise (Mohs 3.5) but is distinguished by its refractive index (1.586-1.605 vs turquoise approximately 1.61-1.65) and by a hot needle test (dye smells synthetic vs turquoise has a specific mineral odour). GIA and any qualified gemologist distinguishes these immediately (GIA; Wise, 2016, pp. 352-355).

Plastic and resin simulants: Blue or blue-green plastic moulded to resemble turquoise with imitation matrix. Warm to the touch (poor thermal conductivity) unlike turquoise (cool to the touch). Lower specific gravity floats or is notably light relative to equivalent turquoise pieces.

Synthetic turquoise: Produced by the Gilson process and by other methods; appears visually similar to natural turquoise but has a more uniform microstructure visible under magnification. Distinguished by FTIR spectroscopy at major laboratories.

Care requirements for turquoise

Turquoise's porous structure makes it particularly susceptible to colour change from absorbed substances. Oils from skin cause gradual greening of the matrix. Perfumes, cosmetics, and cleaning products penetrate and alter colour. Ultrasonic and steam cleaners are damaging. Clean only with a slightly damp soft cloth, no solvents. Remove turquoise jewellery before swimming, bathing, and applying cosmetics. Store separately from harder gems to prevent scratching (Mohs 5-6). Natural A-grade turquoise requires more careful handling than stabilised material because the sealing resins of stabilised turquoise provide some protection against absorption (GIA; Wise, 2016).