The Amber is a piece of fossilized tree resin and appears in three separate locations in the game.
Apart from its great appreciation as a gemstone since ancient times, Amber is also used as an ingredient for perfumes and as an healing agent in folk medicine. Succinum, the Latin term for Amber, is classified in China as being sweet in taste. The ground powder is swallowed with water or with a concoction of herbs, and said to act like a "settling" or "heavy" sedative.
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.
The Amber reappears during our trials in Sheol, where we find it incorporated in one of the puzzles, pertaining to the Third Test to the Key of the Abyss.
Adam: Amber. Fossil resin.