[null and void]

But a cube would be even more exacting, since the volume of a cube is LxWxH,

My first though as well.

BUT corners and edges chip. A sphere is far more mechanically robust.

[William Tell]

null and void said: "BUT corners and edges chip. A sphere is far more mechanically robust."

Also, the sphere will be more "chemically" robust. That is, some of the chemical processes which would cause deterioration of the object will be acting at the surface of the object. A sphere has the minimum surface area possible for a given volume, thus minimizing the amount of deterioration at the surface.