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The blue is attributed to idiochromatic copper while the green may be the result of either iron impurities (replacing aluminium) or dehydration.The refractive index of turquoise (as measured by sodium light, 589.3 nm) is approximately 1.61 or 1.62; this is a mean value seen as a single reading on a gemological refractometer, owing to the almost invariably polycrystalline nature of turquoise.A reading of 1.61–1.65 (birefringence 0.040, biaxial positive) has been taken from rare single crystals.An absorption spectrum may also be obtained with a hand-held spectroscope, revealing a line at 432 nm and a weak band at 460 nm (this is best seen with strong reflected light). It is rare and valuable in finer grades and has been prized as a gemstone and ornamental stone for thousands of years owing to its unique hue.In recent times, turquoise has been devalued, like most other opaque gems, by the introduction onto the market of treatments, imitations and synthetics. Pliny the Elder referred to the mineral as callais and the Aztecs knew it as chalchihuitl.Iran also has turquoise mines in Semnan and Kerman provinces.
Despite its low hardness relative to other gems, turquoise takes a good polish.X-ray diffraction testing shows its crystal system to be triclinic.and greater porosity; these properties are dependent on grain size.Turquoise may also be peppered with flecks of pyrite or interspersed with dark, spidery limonite veining.As a secondary mineral, turquoise forms by the action of percolating acidic aqueous solutions during the weathering and oxidation of preexisting minerals.