An idea that I return to often is that of a pierced window through which one can see semi-precious material. In this case we have a sort of domed gunmetal cage, containing a hemisphere of mother of pearl. It's a kind of gem 'oubliette' - but is the stone imprisoned, trapped, or being revealed? It's quite a compelling formula - the material being just out of reach, tempting, even forbidden! But it also allows the different qualities of light as reflected from different surfaces to play together in the most direct way possible.