Topaz or opal.  This stone resembled an inverted face of a man.  Designated for the Tribe of Dan because in his blessing from Yaakov he was compared to a snake which bites the heel of of the horse, which causes its rider (the enemy) to fall backwards (Bereishit 49:17).  The Tribe of Dan was always pursuing its enemies with their faces always turned upward towards Heaven.  Leshem corresponds to the northern city in Eretz Yisrael that Dan conquered (Yehoshua 19:47).