The Garnet appears in three separate locations in the game.
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.
Garnets are a group of silicate minerals that have been used since the Bronze Age as abrasives and gemstones. While occurring in all colours, red garnets in particular were the most commonly used gemstone during the Late Antique Roman period. Additionally, they were often employed in the so-called Cloisonné technique of decorating metalwork.
Adam: A garnet stone I believe.