True opaline glass is a slightly translucent glass hand blown and popular in the 1800s. The French hand blown glass was made by layering glass. As it was hand-blown, it shows no seams. An evident pontil mark (where the glass is separated from the rod) can be rough or polished. There are no identifying marks or signatures on the bottom. Many opaline pieces were gilded and often feature ormalu which is an ornate gilded brass used in edging, bases, or handles. It is quite valuable.
Other companies made similar products called opaline or opalescent including Portieux Vallerysthal which are not as valuable as the original product. Fenton produced a line of opalescent glass that is sometimes referred to as opaline in online auctions and sales sites. Provincial blue, for example, was made in 1988.
The only books on true opaline are in French. To research your piece, you need to hunt around for information online. Find a goblet similar to your own on eBay, etsy, or another sales site. Many sellers identify their wares though you have to double-check everything to verify as so many get it wrong. This will take some time but you will probably find what you are looking for if you are patient and diligent in your search.