Parshas Shemos: There is a Prisha(YD:264) which brings a fascinating medrash on why we use a metal knife for bris milah as opposed to a stone which was used previously(Tzipporah used a stone,sefer Yehoshua).
He explains when Dovid Hamelech threw the stone at Goliath a conversation between the stone and Goliath’s armor was transpiring. The stone said let me penetrate your armor, but the armor said my job is to protect. The stone said if you give me this honor then I will give you a honor that will last in every generation and that is bris milah.