Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminum. It is rare and valuable in finer grades and been prized as a gem and ornamental stone for thousands of years owing to its unique hue. In recent times turquoise, like most other opaque gems, has been devalued by the introduction of treatments, imitations, and synthetics onto the market, some difficult to detect even by experts. The substance has been known by many names, but the word turquoise was derived around 16th century from the French language either from the word for Turkish or dark-blue stone. It does not occur in Turkey but it traded at Turkish bazaars to Venetian merchants who brought it to Europe. The color, however, has been employed extensively in the decorative tiles adorning Turkish places of worship and homes for hundreds of years, beginning with the Seljuk, or was derived from the color of the Mediterranean Sea on the southern Turkish coast and the association quite possibly has caused the name to take root.