This nicely worked key can be found inside the room with the floating Siren figure, she's wearing the item as a necklace. Be wary enough to put on the Earplugs before entering the room as she emanates a humming noise that will inflict continuous physical damage.
This specific key is used for opening the small Chapel in Raysiel's Tower, allowing us to deal with the puzzle surrounding the seven coloured gems from the stained glass window.
Adam: Nicely worked silver key.
Could this be another one of those more or less subtle references that we elaborate on in this article?