This particular jewel can be picked from the exquisite stained glass window in the small Chapel in Raysiel's Tower. Along with six other gems, it has to be correctly placed according to the ROYGBIV colour spectrum on the star that has been etched on the floor. Completion of this riddle will yield us the Rainbow-coloured Key which is used for opening the door inside the Nightingale Floor Room, allowing us to retrieve the Coloured Gem from a small casket, required to access the shimmering walkway in the main hall of the Room of Riddles.
Turquoise is an opaque, blue-to-green mineral that is a hydrated phosphate of copper and aluminium, with the chemical formula CuAl6(PO4)4(OH)8·4H2. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue, though has been significantly devaluated due to the increasing introduction of imitations.
The substance has been known by many names, but the word turquoise dates to the 17th century and is derived from the French turques for "Turks" because the mineral was first brought to Europe from Turkey, from mines in the historical Khorasan Province of Persia. Pliny the Elder referred to the mineral as callais and the Aztecs knew it as chalchihuitl, literally meaning "heart of the earth" and used in reference to a precious green stone such as Jade or Turquoise.
The Aztecs inlaid turquoise, together with gold, quartz, malachite, jet, jade, coral, and shells, into provocative (and presumably ceremonial) mosaic objects such as masks (some with a human skull as their base), knives, and shields. Natural resins, bitumen and wax were used to bond the turquoise to the objects' base material; this was usually wood, but bone and shell were also used.
In Persia, turquoise was the de facto national stone for millennia, extensively used to decorate objects (from turbans to bridles), mosques and other important buildings. The Persian style and use of turquoise was later brought to India following the establishment of the Mughal Empire there, its influence seen in high purity gold jewellery (together with ruby and diamond) and in such buildings as the Taj Mahal. Persian turquoise was often engraved with devotional words in Arabic script which was then inlaid with gold.
The Egyptian use of turquoise stretches back as far as the First Dynasty and possibly earlier; however, probably the most well-known pieces incorporating the gem are those recovered from Tutankhamun's tomb, most notably the Pharaoh's iconic burial mask which was liberally inlaid with the stone. It also adorned rings and great sweeping necklaces called pectorals. Set in gold, the gem was fashioned into beads, used as inlay, and often carved in a scarab motif, accompanied by carnelian, lapis lazuli, and in later pieces, coloured glass. Turquoise, associated with the goddess Hathor, was so liked by the Ancient Egyptians that it became (arguably) the first gemstone to be imitated, the fair structure created by an artificial glazed ceramic product known as faience.