The silver bows vanish when they aren't needed and reappear once they are. They are also magically strong and springy, but can still be broken as seen in The Last Olympian. Likewise it's possible for the quivers to ran out of arrows, as did Thalia did in The Lost Hero. As with all things used by the Hunters, the bows look silver. It is unknown if the bows are made from some kind of special form of silver as silver is not listed as a precious metal when describing the different kinds. While the silver arrows are able to kill lycanthropes, regular silver shouldn't work on monsters.