My own experience of finding amberina at boot fairs and even antiques markets is limited to a single piece in the shape of a goblet, which does show the shading we've been discussing.
As a type of art glass, it has it's origins in the States where it was patented. so the books say, sometime in the 1880's, and probably that's where almost all was made, and that's no doubt the reason we rarely see it in the U.K.
According to Harold Newman's dictionary - he says that 1883 The New England Co licensed Sowerby to make 'Pressed Amberina' - whatever that may be.
Amberina has two general forms........the more usual is where the ruby occurs in the top half and then, apparently, there is 'Reverse Amberina' where the ruby colour forms the lower part of the object. In it's most desirable form - when the re-striking is for longer than usual - the ruby turns to deep fuchsia, and there's an opaque form apparently, which is described as 'plated amberina'.
Normally, the non-ruby portion should apparently look more akin to light amber/straw.
Ray & Lee Grover's book 'Art Glass Nouveau' - shows some very attractive pieces - none of which helps us with this piece, unfortunately.