Topic: Could be my earliest item of glass.

[flying free]

I think the stem has been blown in opalescent glass then cased in clear.
As I say, I don't know anything about the making of these types of very old items but I guess it could haeve an applied bowl and an applied foot i.e. made in three pieces, with the stem being made decoratively and deliberately opalescent - in much the same way a goblet or footed bowl might have a multi spiral twist stem maybe is what I'm thinking.

m

[Ekimp]

My money is on clouding. In your second photograph the clouding (or whatever) looks a bit patchy. If you have a look with a x10 loupe it should be clear if it’s just clouding on the inside surface.

[flying free]

I agree with Cagney and Ekimp that this is a possible answer - and I know nothing about this era of glass. 
However, just to add that opalescence in glass does actually look 'streaky' or 'patchy' - it isn't a uniform colour if you see what I mean.

[Keith Mick]

Hollow blown decanter stoppers have been known to mist internally. I don't think this is deliberate.