regret I can't help either - other than to suggest this shape is not typical of European art deco. If you look at most of the major factories which produced what we term art deco style - during the '20's and 30's - most designs are chunky, squat and without this sort of stretched neck effect. Whatever the true origins of your piece, this 'onion' shape is more organic and has its origins I would hazard a guess, more in the art nouveau period than deco, and looks rather Islamic in style. I understand that Persian (Iraq now) designs, in turn, came from the Chinese region.
As for the pitted effect on the surface, I think the ceramics collectors call this the 'orange peel effect' - created during the glazing process when manufacturing earthenware, but with glass there is a possibility it happens due to temperature problems when blowing into a mould. Perhaps too cold or too hot maybe, thus preventing the molten glass from solidifying/flowing uniformly - but I am really only speculating here.
I assume you are using the word amethyst because when held up to strong light, the black becomes amethyst. I understand that this 'black/amethyst' effect was an early attempt to produce a black glass - see here for a little info from Ivo re real black glass -
http://www.glassmessages.com/index.php/topic,10322.msg81990.html#msg81990.
The fact that your piece shows amethyst may well put it back into the C19 rather than the C20, and might possibly indicate an eastern European origin, but again only speculation.
All of which helps not a jot as to the origins of your piece, so we are still waiting for an expert
