Turquoise

Turquoise is a hydrated copper aluminium phosphate mineral famous for its distinctive robin's-egg blue to blue-green color. Copper produces the blue, while iron shifts the color toward green; it is almost always opaque.

Many specimens display a web or veining of host rock called matrix (often brown, black or tan), and stones may be described as spiderweb turquoise when the matrix forms a fine network. It is one of the oldest gemstones known, used in Egypt, Persia, Mesoamerica and by Native American peoples.

Because natural high-grade turquoise is porous and relatively soft, much commercial material is stabilized or treated to improve durability and color.

Turquoise forms as a secondary mineral in arid regions where copper-bearing groundwater percolates through aluminium-rich, phosphate-bearing host rocks (often altered volcanic or sedimentary rock). It precipitates in cracks, veins and cavities near the surface, frequently associated with copper deposits.

This weathering and replacement process requires copper, aluminium, phosphorus and water, which is why turquoise is typically found in dry, copper-mining districts. Famous sources include Iran (Nishapur), the southwestern USA (Arizona, Nevada, New Mexico), China, Egypt's Sinai, and Mexico.

Turquoise is opaque, sky blue to green, often with dark matrix veining, and has a waxy to subvitreous luster. Hardness is 5-6 (variable), so it may be scratched by a steel knife; the streak is white to pale greenish. Specific gravity is moderate (~2.6-2.9).

Look-alikes and imitations: Dyed howlite and magnesite are common fakes (howlite is softer and shows gray veining under dye). Chrysocolla is softer and often translucent. Reconstituted or block turquoise (powder bonded with resin) and plastic imitations are widespread. A hot-needle test on plastic, examining matrix naturalness, and checking for unnaturally uniform color help reveal fakes; lab testing confirms treatment.

Turquoise is one of the most important ornamental gemstones, used in rings, necklaces, inlay, beads and especially traditional Native American and Persian silver jewelry. Spiderweb and even sky-blue Persian and Sleeping Beauty material are highly prized.

Much turquoise is stabilized (resin-impregnated) to harden porous stone; lower grades may be dyed or reconstituted, which affects value. Natural, untreated high-grade turquoise commands premium prices.

Culturally it is a December birthstone and a symbol of protection and good fortune across many traditions; metaphysical claims of healing are not scientifically supported.