thanks m - I can see (I think) that the 'stem' is clear over ruby, but my eyes seemed to be telling me the bowl was the opposite - such that the engraving was creating a clear image - but I'm wrong then?

I thought, typically, that colours were cut to clear to create a 'white' appearance to the engraved decoration.
It must just be that the matte surface reflects light differently and gives an illusion of whiteness, with this piece.