Kitkat, that was good information, but the mark "ster" is almost exclusively an American mark. The mark "ster" is used by silversmiths such as James Avery, Jeep Collins, Mignon Fauget, ect. Now I have seen the mark "opus" before, but have never been able to locate the silversmith associated with it. I did a search on the mark and found a defunct Ebay listing for a similar tiger eye ring with the "opus" mark, identified as Canadian, with no other information given. However, Canadian silversmiths have traditionally used "pseudo silver" hallmarks to give the impression of English marks.
All of that being said, during the last twenty years or so, many hallmarks have become quite variable. Most do use the .925 and with the advent of internet marketing, less and less is the country of origin being stressed. Instead the stress is on the country these pieces are being marketed to, namely, the US.
p.s. Welcome to Treasure Net!