In the very basic sense, a sphere is a 3-D circle. And in tombstone iconography, the circle usually represents the unending circle of life; eternity. Earlier today I came across a quote attributed to Joseph Campbell, author of The Power of Myth, that I think truly explains the symbolism of the circle -- even those in 3-D.
The circle on the other hand, represents totality. Everything within a circle is one thing, which is encircled, enframed. That would be the spatial aspect. But the temporal aspect of the circle is that you leave, go somewhere, and always come back. God is the alpha and the omega, the source and the end. The circle suggests immediately a completed totality, whether in time or in space.The totality of one's life. The totality of death... Totality and eternity. Yep, that's it exactly.
[Photo © 2013 S. Lincecum. Taken at Alta Vista Cemetery in Gainesville, Hall County, Georgia.]