Hi Amanda it looks to me like an item marked in the UK. The S&W is what's known as the sponsor mark, and it's the initials of the firm or indiviual who submitted the piece for marking.
the 9 then 375 mark is the gold standard. 9ct or 375 (parts per 1000) or is 37.5% gold (mainly British Commonwealth countries. Not recognized in USA. Hard wearing.)
The light green colored stone sounds interesting. Get it checked out, you never know, it could be an emerald.
