Exodus 28:11-12, 21

With the work of an engraver in stone, like the engravings of a signet, shalt thou engrave the two stones, according to the names of the children of Israel: thou shalt make them to be inclosed in settings of gold. And thou shalt put the two stones upon the shoulder-pieces of the ephod, to be stones of memorial for the children of Israel: and Aaron shall bear their names before Jehovah upon his two shoulders for a memorial.

And the stones shall be according to the names of the children of Israel, twelve, according to their names; like the engravings of a signet, every one according to his name, they shall be for the twelve tribes.