It's made from silver and is covered in some script, specifically magical runes that according to Rebecca, indicate a form of magic connected to sound or lack thereof, giving us an implication to its use:
by wearing the Armlet, we're able to traverse the (otherwise creaking) Nightingale Floor in Raysiel's Tower silently and unharmed, allowing us to retrieve the Rainbow Gem and a Green Potion from a small chest. It should be noted that the second door in the Nightingale Room requires the Rainbow Key to open, which can be gathered from the small Chapel by arranging the seven gems from the stained glass window according to the colour spectrum.
Adam: The floor chimes when we stand on it.
Nightingale floors, or uguisubari (鴬張り), were a type of floor often encountered in Japanese temples and palaces, the most famous example being Nijo Castle, in Kyoto, Japan. They were specifically designed to make a chirping sound when walked upon, thus serving as a security device that would assure no one could sneak through the corridors undetected. The floors are named after the Japanese bush warbler, or uguisu, a type of bushtit or nightingale native to Japan.