Hourglass symbolism
In addition to time-related themes, the hourglass is synonymous with cycles and balance.
Energy passes between the two sides of the hourglass just as the energies of our world are contained by the atmosphere and crust. All of the natural processes and cycles occur there (not including what happens in space, of course), which gives us a greater sense of relation with our environment. This also forces us to realize our roles in the natural cycles happening around us.
Ancient alchemists recognized the concept of balance in the hourglass. Its very shape is made up of triangles balancing each other out. Alchemists interpreted these triangles as representing two aspects of nature: the upper being the sky and the lower equating with Earth.