While it may appear to be a walnut or some other type of nut, in reality it is really a terracotta ball with a slash through it, cast in bronze. Fontana concerned himself with the potential contained within matter, even if an inanimate substance. It was fascinating to him to see something "dead" but still know it had properties and potential energy, etc. The slash across the balls of terracotta symbolizes his desire to make the inert come alive.
In my opinion, it seems contradictory, to take a piece of clay and slash it to release it. It is when he casts it in bronze that the original purpose/message is lost on me. Casting something in bronze seems to be such a form of containing it, stifling it within a hard shell. If anything was ever going to get out of that softer original matter, it certainly has no chance now.