It was the base and the top parts of the stem I was looking at - there's room for a decent bubble in either end. It all flares out nicely. In cut but solid stems, the base and top parts are normally smaller and counld'nt (all the right letters and punctuation - not necessarily in the right order

) accomodate a bubble. And if they're properly sealed they won't let dirt in.
The weight comes from the thick "bowl" part. It looks very top heavy and I'm reallly amazed it's survived as long as it has.