Whiskey glass but that's "globular" at base, looks unstable but isn't? Need for a present.

I recall seeing these years ago, it's basically a heavy glass, err, vessel but not with a flat base. It's sort of tear shaped with the top cut off.

So when you put it down and tip it slightly, it rocks about then settles (in science terms, the act of tilting, raises the centre of gravity)

Anyone know where I can get these?