This is a pretty hefty piece. I assume it is Murano, although I am not certain. (help here would be useful).
But that isn't the point of this:
If you look at the base in particular, you can see a line right through the centre, this isn't a crack, it seems as though when the "lump" of glass (I KNOW gob may be the correct technical term, but it just sounds very rude.....
) was put into the mould they hadn't centred it properly and so added in another lump to fill the gap caused. The clear casing is dramatically thicker on one side than the other.
Now in all other respects this is a nice piece of glass, good heft, nicely finished, aventurine in the central area, would it be usual for a good maker to just chuck in another lump if there wasn't enough?
side view
http://glassgallery.yobunny.org.uk/displayimage.php?pos=-5631 top view
http://glassgallery.yobunny.org.uk/displayimage.php?pos=-5629base
http://glassgallery.yobunny.org.uk/displayimage.php?pos=-5630