thanks Marie. The fact that decoration on this glass is cut, doesn't change my comments about date, unfortunately.
I mentioned petal moulding because that is a well documented form of moulded decoration from the Georgian period, seen often on rummers and some ale glasses after 1800 ish - and is quite distinctive with petal like panels bordered by a raised piping (bit like ceramics).
The gadget or shears mark (which appears to be on your glass), is not seen prior to 1860, and usually on lesser quality pieces - prior to this date a ground/polished pontil depression is more usual (or, of course, a snapped pontil scar) - I expect you are aware of what these look like.
Morris and Hajdamach are very good books (which I'd always recommend), but not of real use if you wanted information on drinking glasses, especially. Collectors of drinking glasses are mostly interested in pieces from the period 1700 to about 1830 - usually classed as C18 glasses, which includes the creme de la creme of innovation and design (and the most expensive - they're a very snobby bunch you know

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But that period did produce the very best of lead drinking glasses - and what came after very much needs careful sorting to find the few good pieces.
Depending on how serious you wish to be about collecting, there are several books that cover drinking glasses and table glass of the C18 - early C19 ............
Bickerton's - 'Eighteenth Century English Drinking Glasses - An Illustrated Guide' (Antique Collectors Club)
E. Barrington Haynes - 'Glass Through the Ages' .........a cheap paperback, but very good.
G. Bernard Hughes - 'English, Scottish and Irish Table Glass' - very heavy and great door-stop if you decide you don't like it, but a good read which is something you can't say of that many glass books.
I'm not aware of anything specifically on later C19 drinking glasses - the Victorians were very much copyists, and perhaps the excesses of the Hanoverians had disappeared and there was nothing new to invent.
Unfortunately, ebay is full of glass sellers who think every piece is Georgian - and it can be difficult (on the screen) to tell the difference between original and copy.
If your impurities are on the surface, try cleaning viorously, otherwise I can't really say - some tavern glasses can include dirt and bubbles.
It's possible to fill a book with suggestions and thoughts on what to look for and what to ignore, but visiting antique centres and glass fairs is a cheap and easy way to see this sort of material in the flesh (provided you don't drop one, or flick them too hard) - but at the end of the day it a case of time and learning plus reading the books.
Good lesson is not to part with pieces until you are good enough to know they aren't something better than you first thought

Your pix might be improved by sharper focus and a background contrast of dark grey or black - but clear glass is never easy to photograph - we have people here who are good and may give their advice.
