Four Rivers

Picture is courtesy of "Vincent44" who published it on Flickr. 
Outside the circle are the Hebrew names of the four rivers that were in the Garden of Eden: 

The name of the first is Pishon, which goes round about all the land of Havilah where there is gold. The name of the second river is the Gihon; it is the one that winds all through the land of Cush. "The name of the third river is the Tigris; it is the one that flows east of Asshur. The fourth river is the Euphrates. 

(genesis 12)

The circle gives the user of this kabbalistic amulet protection from all directions and the same goes for the Star of David.