Arizona State in southwest USA is one of the main producing areas of high-quality turquoises. Natural, Zachery-treated and plastic injection processed Sleeping Beauty turquoises were investigated with the purpose of analyzing their gemological and mineralogical characteristics and explaining the causes of their beautiful color and high hardness. These characteristics were studied by such methods as conventional gemological tests, ultraviolet-visible spectrophotometry, XRD, infrared spectrum, microscopic observation, and WDXRF analysis. The results confirm that, compared with turquoises from other producing areas, the Sleeping Beauty turquoise from the USA has more compact structure and higher Cu²⁺ content, which can explain its higher hardness and purer color. Zachary and injecting process-treated turquoise has more compact and stable structure. WDXRF test of Zachary-treated turquoise shows a visible increase of K element. The infrared spectrum of plastic injection-treated turquoise shows a characteristic weak absorption band at 1 735 cm^-1, caused by the ν(C=O) stretching vibration of the organic compound.