In Tantric Buddhist symbolism, the physical shape of the vajra has certain meanings.
The bulb in the center of the vajra is said to represent the ultimate emptiness (sunyata) that underlies all things.
On each side of the bulb we can see a lotus with eight petals.
The lotus on one side represents the every day phenomenal world (world of forms around us, in samsara), while the lotus on the other side represents the noumenal (divine) world of nirvana.