What is it for?

This is an art installation at a roundabout near where I work. There isn't any label or plaque explaining what it means and/or represents, so one just has to guess. It is a metal cage in the shape of a column, which is filled with uniform rocks in layers with ceramic balls painted with different patterns. It is placed outside an express Sainsbury's Supermarket in an area that has excellent transport links, buses tube and road.
So, any ideas?