This pendant is a version of the Vesica Piscis. The diameter of the outer circle is approximately 1.25 inches. The Vesica Piscis figures prominently in many philosophic traditions and is one of the ancient symbols of Sacred Geometry. Contained within the symbol are the mathematical properties of 1, 2, 3, and the square roots of 2, 3, and 5.
To the Pythagoreans, this symbol was refered to as the Dyad and represents the one separating to become two. While symbolizing balance and harmony, there is the hidden desire to return to the one. So there is a certain dynamism to the balance that implies motion.
Going futher, the symbol can be seen as the two parts of male and female in union with the centers of each circle touching the circumference of the other.
Disclaimer: you need to supply your own chain.