That’s just beautiful. It’s art nouveau, fine silver overlaid on glass, and carries one of Alvin’s marks, originally founded as the Alvin Manufacturing Company in 1886 in Irvington New Jersey. That year they patented a process for depositing pure silver onto metallic and non-metallic surfaces which could then be cut away to expose the base (crystal glass or whatever) in the style of pierced work. Around 1893 the company changed name to the Alvin-Beiderhase Company and then moved to Sag Harbour, long Island a couple of years later.
In later times they traded under other names but that appears to be an early Alvin mark from around 1900 plus or minus a little. Like this one:
Original stopper too, which often isn't the case. I wish it was mine, you lucky dog!
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Well, I know what kind of price tag you might expect to see on it in a fancy-pants antique shop. I would think probably north of $500 and perhaps around $700 if there's no chipping of the glass or damage to the overlay. That won't be what a dealer would offer you though. Personally, I would get it appraised at an auction house and take their advice on saleability and reserve price. Ebay would be a lottery in which you may or may not do better.