Is it completely flat-flat, (ie. is it polished?) or does it have any wavy rings around the surface? (ie. flattened on a marver).
Is there any age-related wear around the part where it would meet the surface it sits on?
Sometimes, if it only sits on a very fine "outer ring" of glass, you might need to take a fingernail around the edge to feel it, rather than see it.
