From what I can tell, it's made of cast metal which was then plated. From the reference pictures I have been able to find online, it appears to be a very close match to the ones used on the show.
However, I'm not exactly an x-files prop officionado, so I'm not sure if it's accurate to one season or another.
I have seen some bad copies of this badge floating around as promotional items. Some of them say something like "Dept of the X-Files" on them. Pretty lame, if you ask me.
I'm hoping to eventualy put together a leather wallet that will contain the shield and the two paper inserts that are featured on the show.