I can see the rounded edges !! Not sharp and distinct like the stopper !! The stopper is not even made with the same form as Waterford !! Do an internet search and you can see the differences in your decanter/stopper and those made by Waterford !!

Can you take a close up of just that part !!

They are not smooth

I mean the edges of the diamond pattern !! Cut glass is literally sharp on the edges !! Your decanter appears to be rounded and smooth !! Just feel the difference between the stoppers edges and the decanter !! Or you can just look at it !! I do not think the decanter is Waterford !! You can also tap the decanter with another glass !! Flint crystal that Waterford is known for will have a clear, melodic, bell like ring !!