A long time ago, when i was a kid/teen, I watched a movie about knights, and battles among them.
I really don't remember anything about the story (other than it was fiction, rather than Historical), but what i remember very well, is that some character looks for a magic Jewel (or Stone) which when you put it in the mouth turns you invisible.
I also kind of remember that the stone/jewel is found on a statue's mouth. And the character who finds it gets an obvious advantage against their enemies.
I estimate is a movie from late 80's , early 90's